The 12-year-old Museum of African Art will officially become part of the Smithsonian Institution next Monday.

"Under the Smithsonian we have the potential to become a major display center of African art in the world," Warren M. Robbins, founder and director of the museum, said yesterday. Currently the museum is a nonprofit, tax-exempt corporation run by a 35-member board of trustees.

Robbins, who proposed the merger to Smithsonian Secretary S. Dillon Ripley over three years ago, expects the museum to attract more shows and donors, and foresees more gallery space in a proposed new building.

A sum of $500,000 has been authorized but not yet appropriated by Congress for a planning study of the South Quadrangle Project, a Smithsonian spokesman said yesterday.

The long-range plan includes a gallery of Oriental art adjoining the Freer Gallery, a new museum of African art adjoining the Arts and Industries building of the Smithsonia , and a rare-book library.

All parts of the Museum of African Art - including the gallery, store, library and other facilities located in nine historic row houses on A Street NE - will now become property of the Smithsonian.

The museum, tightly filled with more than 8,500 artifacts, will soon be receiving the late philanthropist Samuel Rubin's collection of 80 African art pieces, Robbins said yesterday. And another collection of 700 art objects and some 1,000 books on African culture and art from the collection of anthropologist Melville Herskovits will be permanently loaned to the museum, Robbins added.