Genistein is one of the most abundant isoflavones in soy. The effects of genistein on cholesterol synthesis and fatty acid oxidation have been well documented, but the effect of genistein on fatty acid synthesis remains unclear. Thus, we investigated the effect of genistein on fatty acid synthase (FAS) expressions in HepG2 cells. In HepG2 cells treated with 10 micromol/L genistein, mRNA and protein expressions of FAS, as well as FAS activity, were significantly decreased. The promoter region of FAS contains binding sites for the transcription factor called sterol regulated element binding protein 1 (SREBP-1); SREBP-1 must be processed by site-1 (S1P) and site-2 proteases to be activated. We also investigated the effects of genistein on S1P, SREBP-1 expression, and subsequent SREBP-1 processing by S1P in HepG2 cells. Genistein reduced the expression of S1P and the processing of SREBP-1 but did not change the expression of SREBP-1 mRNA. SREBP-1 is also a transcription factor for lipogenic genes, such as stearoyl coenzyme-A desaturase1 (SCD1), glycerol-3-phosphate acyltransferase (GPAT), and acetyl-CoA carboxylase (ACC)1, and ACC2. Genistein also significantly inhibited the expression of these lipogenic genes. Thus, genistein treatment of HepG2 cells decreased the expression of lipogenic genes such as FAS, SCD1, GPAT, and ACC, which is, at least in part, mediated through the downregulation of S1P expression and subsequent SREBP-1 proteolytic cleavage.