Abstract: Objective To study the effects of adenosine triphosphate (ATP) on the proliferation of human squamous esophageal carcinoma Eca-109 and hepatoma SMMC-7721 cells in vitro and the underlying mechanism. Methods MTT assay was used to determine the proliferation of tumor cells. The AO/EB double stained cells were observed under fluorescence microscope to observe morphological changes. The effects of ATP on the cell cycle, apoptotic rate and apoptosis-related protein were detected by flow cytometry. Results ATP produced inhibitory effects on Eca-109 and SMMC-7721 cells at the concentration between 0.01~0.3 mmol/L. Exposed to 0.3mmol/L ATP for 72h, some of SMMC-7721 cells displayed morphological changes of apoptosis, but Eca-109 cells did not show the characteristics of apoptosis markedly. There was no significant change in the apoptotic rate and apoptosis-related protein of the two tumor cell lines treated with ATP 0.03, 0.1 and 0.3mmol/L for 72h respectively. The proportion of Eca-109 cells in G0/G1-phase of cell cycle was significantly increased, meanwhile the proportion of Eca-109 cells in S-phase and proliferation index value was significantly decreased by treatment with 0.3mmol/L ATP. Conclusion ATP inhibited Eca-109 cell proliferation by changing the distribution of cell cycle phase, and its mechanism might not related to apoptosis, but for SMMC-7721 cell, the inhibition of cell proliferation induced by ATP was not related to the change in cell cycle.