Objective: To study the effect of nicotine on alkaline phosphatase (ALP) activity of human dental papilla mesenchymal cells in vitro. Methods: Human dental papilla mesenchymal cells were cultured in the presence of nicotine at various concentrations, ALP level of the cells was meseased at different time with an enzyme dynamical method and was expressed as A value at 410 um. Results: After 3day-exposure of the cells to nicotine(g/L) at 0, 0.05, 0.1, 0.2 and 0.4 the A values(per 10 5cells) were 0.64 ? 0.15, 0.61?0.03, 0.47 ? 0.05, 0.44 ? 0.06 and 0.39? 0.03; after 5-day-exposure those were 0.55 ? 0.12, 0.42?0.16, 0.29? 0.10, 0.19 ? 0. 4 and 0.10?0.02; after 7 day exposure those were 0.40? 0.12, 0.31 ? 0.10, 0.26? 0.06, 0.13? 0.02 and 0.09? 0.02, respectively. Conclusion: Nicotine inhibited the alkaline activity of human dental papilla mesenchymal cells in a dose and time dependant manner. Nicotine may play a role on the differentiation of the cells.