At a party in their Brentwood Park home, Mr. and Mrs. Philip Barry of West Los Angeles and New York City announced the engagement of their daughter, Stephanie Anne Thankful, to Mark William Agnew, sone of Mr. and Mrs. William Wilson Agnew, III of West Boxford, Mass.

Miss Barry is a graduate of Marlborough School and Stanford University.

She was presented in 1975 at the Coronet Debutante Ball, the New York Infirmary Cotillion, the International Ball, and at a cocktail party at the home of Alice Roosevelt Longworth. She is the granddaughter of Mrs. Paul A. White of Davenport, Iowa and the late Dr. White, and of Mrs. Philip Barry of Georgetown and the Philip Barry, playwright. Miss Barry's father is president of Philip Barry Productions, affiliated with Osmond Television, and her mother is the actress Patricia Barry.

Mr. Agnew is a graduate of Masxonomet Regional High School where he was chosen by the Salem Evening News as the 1977 student/athlete of the year. He will receive a degree in economics from Stanford University in June. His father is a captain with American Airlines and he is the grandson of Mrs. Richard Rezac of Santa Rosa, Cal. and the late Mr. A. Ben Echols, and of Mrs. William Wilson Agnew, II, of Gainesville, Tx. and the late Dr. Agnew.

The wedding will take place in June.