LOUISVILLE, Ky. (AP) _ The American Legion will for the first time select a Vietnam veteran as its national commander this week.

Delegates to the Legion's 70th annual convention will choose on Thursday between H.F. ''Sparky'' Gierke, a justice of the North Dakota Supreme Court in Bismarck, and Miles Epling, a court clerk in Point Pleasant, W.Va.

Both candidates for the post served in the Vietnam War.

''I think it signifies a passing of additional responsibility to the veterans of our era, but that's been going on for a long time,'' said Gierke, 45, who was in the Army in Vietnam in 1969-70.

Some 20,000 delegates, auxiliary members and guests were expected at the convention of the nation's largest veterans organization. Legionnaires held a parade here Sunday; the main business of the convention was scheduled for Tuesday through Thursday.

Epling, 38, said Sunday that having only Vietnam veterans as candidates for commander of the 2.8-million-member organization is a natural progression.

''A lot of the Vietnam veterans are getting to the point where they're beginning to join organizations,'' said Epling, who served as a Marine in Vietnam in 1969.

The Legion does not keep records on the subject, but estimates of the number of Vietnam veterans in the group range from 400,000 to 800,000. World War II veterans still make up the largest single group in the organization.

Four veterans of the Korean War have served as national commender of the Legion, including the current leader, John P. ''Jake'' Comer of Massachusetts.