Dr. Willard F. Libby, 71, who won the Nobel Prize in chemistry for his method of using radioactivity to determine the age of ancient objects, died Monday night at the UCLA Medical Center in Los Angeles.

In additon to his "atomic clock" or "atomic calendar," which has become indispensable in archeology and ancient history, Dr. Libby -- known to friends as "Wild Bill" -- was a leader in the effort to harness nuclear energy for peaceful purposes.

He was a member of the Manhattan Project, which built the atomic bomb, and served in the 1950s on the U.S. Atomic Energy Commision, where he was credited with being a moving force in the Atoms for Peace program.

Reconganized for his wide-ranging imagination and curiosity, Dr. Libby was a professor of chemistry at UCLA for many years and also headed the university's institute of geophysics before retiring in 1976.

His death, which followed a brief illness, was attributed to a blood clot in the lung, complicated by pneumonia.

Dr. Libby's famed "atomic calendar" depends essentially on the ability of scientists to measure precisely the slow decay of Carbon-14, a radioactive isotope of carbon found in many acient objects.

Used by geologists, geoghysicists and by specialists in many areas in addition to archeology, the Carbon-14 dating technique made possible many discoveries about ancient man and his environment over a period going back 50,000 years.

First through theoretical argument, then through experiment, Dr. Libby demonstrated that radioactive carbon would be incorporated in all living matter in relatively fixed concentrations.

When the plant or animal dies, it can no longer asimilate the carbon isotope, and the existing concentration slowly declines. Through the process of radioactive decay, half of the Carbon-14 concentration is lost every 5,750 years. By using a Geiger counter to measure radiation emitted by a sample, scientists can tell just how long the decay has continued, and hence measure the sample's age.

A 6-foot-2 native of Grand Valley, Colo., Dr. Libby was raised on his family's fruit ranch near Sebastopol, Calif., and earned bachelor's and PhD degrees in chemistry at the University of California at Berkeley.

After 10 years of teaching at Berkely, he joined the Manhattan Project, helping develop the gaseous diffusion method of separating out the fissionable isotope of uranium.

The work that led to the Nobel Prize was carried out while on the university of Chicago faculty after the war. He was appointed to the Atomic Energy Commission in 1954, for a five-year term.

Afterward, he returned to California as a chemistry professor at UCLA, where he was known for his eagerness to combine elementary teaching with advanced research.

After the 1968 election, there were indications that Dr. Libby, who had supported Richard Nixon, might be named the new president's science adviser. The reports evoked strong private protests from many scientists who complained that Dr. Libby was too conservative politically to mobilize the nation's scientific community.

In addition to the 1960 Nobel prize, Dr. Libby, who lived in Santa Monica, held a wide variety of scholarly and scientific honors and awards, including a dozen honorary degrees, the Joseph Priestly medal and the Albert Einstein medal.

Survivors include his wife, Dr. Leona Marshall Libby, an internationally known scientist, and two daughters, two brothers and a sister.