Objectives. Gastric cancer is one of the leading causes of death in Taiwan and Solanum nigrum is one of the medicinal plants used in cancer therapy. However, there is no available information that addresses the mechanism by which Solanum nigrum affects human stomach cancer cells. Methods. We examined the dose effect of crude Solanum nigrum extract on the degree of N-acetylation of 2-aminofluorene (2-AF) in SC-M1 cell cytosol and intact cell cultures by acetyl Co-A recycling mixture assay and high pressure liquid chromatography (HPLC). Results. The crude extract of Solanum nigrum reducesed NAT activity in SC-M1 cytosol and intact cell cultures. Conclusions. Solanum nigrum affects malignant diseases by reducing NAT activity in SC-M1 cells.