Sirt1, a NAD(+)-dependent histone deacetylase, may regulate senescence, metabolism, and apoptosis. In this study, primary pig preadipocytes were cultured in DMEM/F12 medium containing 10% fetal bovine serum (FBS) with or without reagents affecting Sirt1 activity. The adipocyte differentiation process was visualized by light microscopy after Oil red O staining. Proliferation and differentiation of preadipocytes was measured using methylthiazolyldiphenyl-tetrazolium bromide (MTT) and Oil red O extraction. Expression of Sirt1, FoxO1, and adipocyte specific genes was detected with semi-quantitive RT-PCR. The results showed that Sirt1 mRNA was widely expressed in various pig tissues from different developmental stages. Sirt1 mRNA was expressed throughout the entire differentiation process of pig preadipocytes. Resveratrol significantly increased Sirt1 mRNA expression, but decreased the expression of FoxO1 and adipocyte marker gene PPARgamma2. Resveratrol significantly inhibited pig preadipocyte proliferation and differentiation. Nicotinamide decreased the expression of Sirt1 mRNA, but increased the expression of FoxO1 and adipocyte specific genes. Nicotinamide greatly stimulated the proliferation and differentiation of pig preadipocytes. In conclusion, these results indicate that Sirt1 may modulate the proliferation and differentiation of pig preadipocytes. Sirt1 may down-regulate pig preadipocytes proliferation and differentiation through repression of adipocyte genes or FoxO1.