The Defense Department has refused to reclassify World War II merchant seamen as veterans and declare them eligible for some veterans' benefits.

Tidal W. McCoy, assistant secretary of the Air Force for manpower, reserve affairs and installations, ruled last week that the civilian sailors did not meet criteria in a 1977 law that allowed some civilians to be reclassified as veterans.

Although the merchant fleet often sailed in combat areas and more than 6,000 civilian seamen lost their lives when ships were torpedoed, McCoy classified them with 23 other groups whose retroactive requests for veteran status have been rejected.

The seamen had no reasonable expectation at the time of being classified as veterans, he found, and unlike their counterparts in the Navy were not required to serve for the duration of the war.

Civilian seamen represent the largest of many groups that served the war effort in one way or another in the early 1940s and were allowed to seek reclassification as veterans. Among those rejected earlier were post exchange accountants, employes of the War Production Board, and military police auxiliaries.

The application of a group called the Women's Air Force Service Pilots (WASPS) was approved, however, and the merchant sailors argued that the mariners met all the criteria applied to the WASPS.

It is not known exactly how many merchant sailors would have been affected by a grant of veteran status. Maritime industry groups who supported their application said what they wanted most was the right to be buried in national cemeteries.

David Leff, director of the Joint Maritime Congress, which sponsored the preparation of the mariners' case, said the decision would be appealed.