Severe Acute Respiratory Syndrome coronavirus (SARS-CoV), cause of the life-threatening atypical pneumonia, infects many organs, such as lung, liver and immune organ, and induces parenchyma cells apoptosis and necrosis. The genome of SARS-CoV, not closely related to any of the previously characterized coronavirus, encodes replicase and four major structural proteins and a number of non-structural proteins. Published studies suggest that some non-structural proteins may play important roles in the replication, virulence and pathogenesis of viruses. Among the potential SARS-CoV non-structural proteins, 3b protein (ORF4) is predicted encoding 154 amino acids, lacking significant similarities to any known proteins. Till now, there is no report about the function of 3b protein. In this study, 3b gene was linked with the EGFP tag at the C- terminus. Through cell cycle analysis, it was found that over-expression of 3b-EGFP protein in Vero, 293 and COS-7 cells could induce cell cycle arrest at G0/G1 phase, and that especially in COS-7 cells, expression of 3b-EGFP was able to induce the increase of sub-G1 phase from 24 h after transfection, which was most obvious at 48 h. The apoptosis induction of 3b fusion protein in COS-7 cells was further confirmed by double cell labeling with 7-AAD and Annexin V, the function of 3b protein inducing cell G0/G1 arrest and apoptosis may provide a new insight for further study on the mechanism of SARS pathogenesis.