Scott Nearing, 100, a radical reformer since the early part of the century and a nationally acclaimed leader of the back-to-the-land movement for the last 50 years, died of pneumonia and dehydration yesterday at his home in Harborside, Me.

A left wing intellectual who seemed to push further left every time the establishment rebuked him, Mr. Nearing was the author of some 50 books that ranged from autobiographical accounts of his radicalism to graceful essays on pacifism and nonviolence as the salvation of the West. He called himself "an outcast from a dying social order."

To most of the public, Mr. Nearing was best known for the 1954 book "Livng the Good Life," which he co-authored with his wife, Helen. At first, the book, which told of the couples' adventures into self-sufficiency and vegetarianism on the Vermont farm they retreated to in 1932, went almost unnoticed with a sale of only 3,000 copies.

In 1970, when it was reissued, a new generation of readers, weary of the despoliation to the land that the Nearings saw coming decades before, bought 170,000 copies and turned the 87-year-old radical into a national celebrity. Mr. Nearing, a committed socialist, refused all royalties from the book. The profits were turned over to the Social Science Institute in Harborside.

Much like Henry David Thoreau, whose "Walden" was a precursor of "Living the Good Life," Mr. Nearing understood that his philosophy attracted many admirers but few imitators.

With wryness, he wrote of his two decades of homesteading in Vermont: "To the credit of Vermont conservatism it must be said that during the two decades of our stay, after innumerable discussions and long, drawn-out arguments on the subject of white flour, white bread, white sugar, pies and pastries, the necessity for eating raw vegetables and the revolting practice of consuming decaying animal carcasses, no native Vermont family of our acquaintance made any noticeable change in its food habits."

Scott Nearing was born Aug. 6, 1883, in Morris Run, Pa., a coal mining town that was ruled by his grandfather. At an early age, noticing the misery of the miners and the few chances they had to escape the teachery of company-town exploitation, he took to books to learn solutions. He discovered Leo Tolstoy.

In "The Making Of A Radical: A Political Autobiography," Mr. Nearing wrote that Tolstoy's "writings and his urgent motto, 'make peace, love one another,' became a guiding light to me in a darkening world. When the leaders of the major civilized western European powers directed their citizens to invade frontiers, overrun villages, desolate cities, and destroy, burn, and murder, I turned more and more to Tolstoy as my counsellor and guide. I read his works and publicized his ideas as best I could. I also endeavored to simplify my life, and eventually became like him a vegetarian, a pacifist and a socialist."

Mr. Nearing was to pay heavily for his views. He earned a doctorate in economics in 1909 at the University of Pennsylvania. In 1914, as a teacher of economics at the Wharton School of Finance, he spoke out and crusaded against the exploitation of child labor. For the audacity of saying it was wrong to pay children 13 cents an hour for mining coal, he was fired by the trustees. The case created a national uproar not only about child labor, but also about the need for academic freedom.

In 1917, he was in trouble again. His anti-war essay, "The Great Madness," in which he attacked American participation in World War I, led to a federal indictment. In a celebrated trial, he was acquitted by the jury but not the establishment. Publishers once glad to take his books now shunned him. Magazine editors sent back his manuscripts.

Years later, he wrote of what he had endured. "From personal experience I can bear witness that war not only negates truth, decency, and human kindness, but brings disaster also to truth-seekers and those who are devoting their energies to social improvement. War is hell. More than that, war drags human beings from their tasks of building and improving, and pushes them en masse into the category of destoyers and killers."

A gifted public speaker who once toured the country debating Clarence Darrow, Mr. Nearing ran for Congress in 1918 against Fiorello LaGuardia. He was defeated, but the left recognized him as one of its leaders. Eugene Debs, the socialist organizer, spoke on his behalf.

For a brief time in the 1920s, Mr. Nearing, after traveling through Russia, joined the Communist Party. It, too, rejected him for his independent thinking. His error was to criticize some of Lenin's dogmas.

Now in his 40s, he asked the questions common to a mid-life crisis: "Where did I belong? How could I classify myself? Was I crazy and were my stand-pat conservative fellow citizens sane? Was I alone sane and they all off the track?"

Following the failure of his marriage to Nellie Seeds, Mr. Nearing became a homesteader in 1932 in Vermont with Helen Knothe, a concert violinist. He was nearly 50, she was 29. They lived in an isolated farm without electricity, telephone or radio. They raised their own crops, built a farmhouse out of stones gathered in the fields, and once a week went foodless for a day. The fasting, Mr. Nearing wrote, was "to give the disgestive system a rest . . . and to occupy our day with activities more significant than eating."

In 1952, with a ski resort crowding them out, the Nearings picked up and moved to another farm in Maine to begin again. He was 70. The couple again built a home with their own hands.

Throughout the 1960s and 1970s, thousands of visitors every year made pilgrimages to the Nearing farm. People came away from Mr. Nearing in much the same way as readers came away from his books. In a 1971 Washington Post review of "The Maple Sugar Book" and "Living the Good Life," a critic wrote that they "caught me as no other books have ever done. In one day my conventional goals were shaken to pieces and a vision took their place."

In addition to his wife of Harborside, Mr. Nearing's survivors include a son by his first marriage, Robert of Troy, Pa., and two grandchildren.