This study was aimed to investigate the effect of arsenic trioxide (As2O3) on proliferation and cell cycle of human Burkitt lymphoma cells and its related molecular mechanism, so as to provide experimental evidence for treatment of Burkitt lymphoma with As2O3. Human Burkitt lymphoma cell line Namalwa was used as the model, the effect of As2O3 on cell proliferation, cell cycle and apoptosis, as well as the expression of cell cycle modulation related genes, including mRNA and protein level, were detected by MTT method, flow cytometry, real-time quantitative PCR (RQ-PCR) and Western blot, respectively. The results showed that the As2O3 inhibited significantly the growth and proliferation of Namalwa cells in concentration-and time-dependent manner. The As2O3 arrested obviously cell cycle of Namalwa cells in G1 phase, and showed significant concentration-effect relationship. The As2O3 induced the apoptosis of Namalwa cells in concentration-and time-dependent manner, downregulated the expression of the important driving genes of cell cycle including Cyclin E and CDK2 in mRNA and protein level, upregulated the expression of the important inhibiting gene of cell cycle-P21 in mRNA and protein level in concentration-dependent manner. It is concluded that As2O3 inhibits significantly the growth and proliferation of Namalwa cells, and the effect was closely relates with its inducing the apoptosis and blocking the cell cycle of Namalwa. The action of blocking cell cycle is closely associated with its downregulating the expression of driving genes of cell cycle-Cyclin E and CDK2, upregulating the expression of the inhibiting gene of cell cycle-P21.