Objective: The purpose of the study was to study the ef ect of Huaier, a traditional Chinese medicine, on the cel cycle adjustment in MOLT4 cel s in vitro. Methods: We used MTT assay to test cel viability, flow cytometry to detect cel cycle and apoptosis and western blot to examine the expression of cel -cycle and apoptotic proteins in MOLT4 cel s induced by Huaier. Results: Huaier could reduce the viability of MOLT4 cel by inducing G1 arrest and apoptosis. The induction of apoptosis after treatment with Huaier for 24 h was demonstrated in a dose- and time-dependent manner by flow cytometry analysis. G1 arrest induced by Huaier was modulated through the increased expression of Cdki proteins (p21cip/waf1 and p27kip1) with a simultaneous decrease in Cdk2, Cdk4, Cdk6, cyclin D1 and cyclin E expression. Huaier also induced Bax and Bcl-2 expression and activation of Caspase-3. Conclusion: It is firstly demonstrated that Huaier can inhibit proliferation of MOLT4 cel s via G1 arrest and apoptosis. These results suggest that Huaier is a cel -cycle anti-cancer drug.