Hepburn is the ninth actor, actress or director honored in the series, which began in 1995 with Marilyn Monroe.
The Audrey Hepburn stamp was delayed a few times and proved more difficult to design than chief stamp designer Terry McCaffrey expected.
"We assumed it would be a very simple project because everyone knows Audrey Hepburn, which they do, but because of that, we have to be very careful in getting the exact shape of everything," he told CBSNews.com.
And the U.S. Postal Service was especially careful, because the Hepburn family wasn't happy with the stamp issued by another country a few years ago.
Michael Deas, the same artist who designed the earlier stamps in the series did this portrait, but while it took more drafts than the others, "we think it's a beautiful portrait, based on a photo taken from Sabrina, one of the early films," said McCaffrey.
The stamp was being dedicated at a ceremony in Los Angeles at the Children's Hospital, one of the charities supported by the Audrey Hepburn Children's Fund. She was also active for many years in work for UNICEF.
Hepburn was awarded the Presidential Medal of Freedom in 1992. The following year, she received a Life Achievement Award from the Screen Actors Guild, and, posthumously, the Jean Hersholt Humanitarian Award, an honorary Academy Award. She died at her home in Switzerland on Jan. 20, 1993, at the age of 63.
"Audrey Hepburn's achievements on the stage and screen alone make her a wonderful addition to our nation's stamp program," said S. David Fineman, chairman of the Postal Service Board of Governors, who will dedicate the stamp. "But we also honor her for her work as an enthusiastic and dedicated advocate for the well being of children around the world."
Sean Ferrer, son of Audrey Hepburn and chairman of the Audrey Hepburn Children's Fund, was scheduled to take part in the dedication, as was actress Jennifer Love Hewitt, a member of the fund's advisory board who played Hepburn in a television movie in 2000 and is starring in an upcoming theatrical film about a teen-ager obsessed with Hepburn, "Why Can't I Be Audrey Hepburn?"
Hepburn was born in Brussels, Belgium, in 1929, the daughter of a Dutch baroness and her British husband. She spent much of her childhood in England, but in 1939 she moved with her mother to the Netherlands. After World War II, Hepburn received a ballet scholarship in London.
She won the Academy Award for best actress for her first major Hollywood film role, playing a princess in "Roman Holiday" (1953). She was also nominated for four other roles: "Sabrina" (1954), "The Nun's Story" (1959), "Breakfast at Tiffany's" (1961) and "Wait Until Dark" (1967).
The margin area of the sheet of stamps shows her in "Breakfast at Tiffany's."
She also received a Tony Award for her performance in the Broadway play "Ondine."
The Audrey Hepburn Children's Fund was founded in 1994 and later established the Audrey Hepburn CARES Team at Children's Hospital Los Angeles, which provides medical and mental health services to suspected victims of child abuse.
Previous stamps in the Legends of Hollywood series were Monroe, James Dean, Humphrey Bogart, Alfred Hitchcock, James Cagney, Edward G. Robinson, Lucille Ball and, last year, Cary Grant.
The Audrey Hepburn stamp will go on sale at post offices outside Los Angeles on Thursday.
By Lloyd A. de Vries