United Artists Corp., which last December wrote off $15 million of losses for the box-office flop, "Heaven's Gate," today wrote off another $29 million.

A spokesman for Transamerica Corp., which owns United Artists, said the $44 million charge will cover all of the film company's investment in the film itself and the costs of advertising and distribution.

"Heaven's Gate," directed by Michael Cimino, was budgeted at $12 million, but Pentagon-size cost overruns pushed the final cost to more than $40 million.

Metro-Goldwyn-Mayer Film Co. has agreed to buy United Artists from Transamerica for $380 million, a transaction that is supposed to be completed in the fall. A Transamerica spokesman said the purchase price "reflects the 'Heaven's Gate,'" losses, reportedly the biggest financial flop in film-making history.

Because of the $29 million write-off, Transamerica said United Artists will lose money in the second quarter. It lost money in the final three months of 1980 when it took the $15 million charge, but made money during the first three months of 1981.

Industry sources lay the blame for the "Heaven's Gate" disaster at the feet of United Artists management, who permitted Cimino, the celebrated director of "The Deerhunter," to continue to spend money for a movie that was critical as well as financial failure.

Transamerica bought United Artists in 1967. Analysts said the film company never fit in the hodge-podge of companies controlled by Transamerica -- including Occidental Life Insurance Co. of California (the company's biggest cash producer), Budget Rent-a-Car, Transamerica Airlines, equipment leasing and manufacturing of industrial machinery.

Last year Transamerica Corp. had sales of $4.4 billion. Financial services and manufacturing accounted for more than 80 percent of its revenues.

Analysts said that James R. Harvey, who became Transamerica's chief executive in January, appears determined to concentrate the company's efforts in those two areas.