This study was aimed to investigate the sensitizing effect of recombinant human PDCD5 (rhPDCD5) protein on chemotherapy of U937 cell line and its mechanism. The flow cytometry was performed to assess the changes of cell apoptosis and cell cycle influenced by rhPDCD5. Hochst 33258 staining was used to observe morphology of the apoptotic cells. The activity change of caspase-3 was detected to analyse the possible mechanisms of rhPDCD5-induced apoptosis. RT-PCR was performed to observe the expression level of drug-resistant genes. The results showed that the percentage of apoptotic cells and the activity of caspase-3 remarkably increased in U937 cells treated with rhPDCD5 combined with chemotherapeutic drug; the cell cycle arrest induced by anti-tumor drug was also enhanced when combined with rhPDCD5; meanwhile, the expression levels of drug-resistant genes were down-regulated in jointly treated U937 cells. It is concluded that the chemosensitizing mechanisms of rhPDCD5 are complex. rhPDCD5 may increase the cytotoxicity of anti-tumor drugs by promoting the caspase-3-related apoptosis, influencing cell cycle, decreasing the expression of drug-resistant genes and reversing drug-resistance.