James B. Cobb, 75, former president of the National Alliance of Postal and Federal Employees and a practicing lawyer in Washington, died of cancer Monday at Howard University Hospital.

He had been a career postal employe for 31 years when he requested optional retirement from the postal service in 1961. At the same time, he became president emeritus of the alliance, which he had headed for eight years.

In 1963, Mr. Cobb was named superintendent of training in the Washington City Post Office, a position he held for five years.

During most of his postal career, he also practiced law with his son, James W. and was active in the law firm of Cobb, Borders and Williams until his death.

Mr. Cobb was born in Durham, N.C. He came here to attend Howard University, and graduated with a bachelor of science degree. He later earned a doctor of jurisprudence degree from Howard's Law School, then was admitted to the D.C. Bar and to practice before the U.S. Supreme Court.

He was a member of Berean Baptist Church where he sang and directed the choir. He was a past master of the Prince Hall Masonic Lodge.

In addition to his son, of Washington, he is survived by his wife, Leona M., of the home in Washington; two daughters, Dr. Bessie Howard, of Washington, and Dr. Joan Wilson, of Potomac; a brother, Dr. Charles E. Cobb and a sister, Mary Genus, both of Washington; another sister, Margaret Allen, of Durham, and 10 grandchildren.