Father Junipero Serra, a Spanish Franciscan friar who established the missions that formed the base for the settlement of California, is appearing on a new 44-cent airmail stamp that comes out Thursday in San Diego, the site of his first mission.

It is the fourth consecutive year that the United States has issued a stamp in honor of a religious figure. There have been more religious stamps during the Reagan presidency than ever before, excluding the annual Christmas issues..

The new airmail stamp is based on a 40-peseta issue put out in 1984 by Spain for Serra. Both stamps bear a portrait of him centered between an outline of the state's southern coast and the Baja California peninsula on the left, and the facade of Mission San Gabriel on the right side.

Serra was born on a farm on the island of Majorca in 1713 and lived his first 36 years there. He entered the Franciscan order at 17 and became a distinguished professor of philosophy and theology and a pulpit orator.

In 1750 he forsook all of this to become a missionary and traveled with a group of Franciscans to Mexico, then a colony of Spain. He worked for nine years with the Indians of Sierra Gorda, then a like number of years crisscrossing central Mexico and preaching.

When Spain began the conquest of upper California in 1769, Serra accompanied the military expedition northward along the coast, and in July of that year founded his San Diego mission. Over the next 15 years, he presided over the founding of eight more missions, among them San Francisco, Santa Clara, San Luis Obispo, San Juan Capistrano and San Carlos in the Monterey-Carmel area.

A total of 21 missions eventually were established, all linked by El Camino Real -- the Royal Road. Serra was a guiding force in the settlement of California.

The 44-cent airmail stamp, designed by Robert Schlecht of Arlington, represents the rate to all destinations except Canada, Mexico, some areas of Central and South America and the Caribbean islands.

The horizontal issue has been produced by gravure in four colors: yellow, magenta, cyan and black. There are four plate numbers on the selvage of each post office pane of 50 stamps.

Collectors of first-day-of-issue cancellations have a Sept. 21 deadline for ordering -- orders must be postmarked by that date.

Collectors affixing stamps to their envelopes, which must be addressed, should send the first-day covers to Customer-Affixed Envelopes, Postmaster, San Diego, Calif. 92199-9991.

Collectors preferring full processing by the Postal Service should send their addressed envelopes to Junipero Serra Stamp, Postmaster, San Diego, Calif. 92199-9992. The cost is 44 cents per stamp affixed to a cover. Personal checks are accepted.

The three other recent stamps honoring religious leaders were a 1982 20-cent commemorative for St. Francis of Assisi, a 1983 20-cent commemorative for Martin Luther, and a 13-cent postal card for Father Frederic Baraga, a Roman Catholic missionary who worked with the Indians of the Great Lakes.