Twelve American journalists, including Michael E. Hill of The Washington Post, will study at Harvard University next fall under Nieman Fellowship.

Hill, 36, is an assistant editor in The Post's Style section. He came to the paper in June 1970 and has been an assistant city editor and assistant sports editor.

He previously worked for five years as a reporter and editor.

The other Nieman Fellows announced yesterday are:

Carlos Aguilar, 33, reporter for KENS-TV, San Antonio, Tex.; Rose M. Economou, 34, producer for WBBM-TV, Chicago; Frank G. Adams, 40, Helena bureau chief of the Great Falls (Mont.) Tribune; Peter J. Almond, 34, reporter for the Cleveland Press; and Gerald M. Boyd, 29, Washington reporters for the St. Louis Post-Dispatch.

Also, David S. Lamb, 40, East Africa bureau chief, the Los Angeles Times; Douglas N. Marlette, 30 editorial cartoonist, the Charlotte (N.C.) Observer; Laurel Shackelford, 33, assistant city editor, The Courier-Journal Louisville, Ky.; Howard S. Shapiro, 31, reporter, The Philadelphia Inquirer James D. Stewart, 33 reporter, The Atlanta Constitution: and Nancy C. Warnecke, 32, photographer, The (Nashville) Tennessean.

The Nieman Foundation, which was established through a bequest from Agnes Wahl Nieman in memory of her husband, Lucius, supports journalists for one year's study at Harvard in a program of their choosing.