Signal transducers and activators of transcription 5 (STAT5) has been shown to be involved in a variety of cellular processes, including survival, proliferation, invasion, angiogenesis and immune evasion and is frequently overexpressed in human solid tumors and blood malignancies. The aim of this study was to investigate the role of STAT5a in colorectal cancer (CRC) progression. We inhibited the expression of STAT5a using lentivirus-mediated artificial microRNA (miRNA) interference in vitro and investigated the viability of CRC cells by CCK-8 assay. We observed the cell viability after treatment with cisplatin (CDDP) or 5-fluorouracil (5-Fu) by CCK-8 assay, and the apoptosis induced by chemotherapy using flow cytometric analysis and Annexin V RFP staining assay. We inhibited the mRNA expression by 54% and the protein expression by 60% of STAT5 by RNA interference targeting STAT5a. Cell viability assays showed that inhibition of STAT5a did not affect the viability of SW1116 cells. However, we found that inhibition of STAT5a restored the sensitivity of SW1116 cells to CDDP and 5-Fu. In additional experiments, we found that inhibition of STAT5a significantly promoted CRC cell apoptosis by CDDP and 5-Fu. In the present study, we found that inhibition of STAT5a promotes apoptosis of CRC cells induced by chemotherapy drugs, such as CDDP or 5-Fu. These results suggest that inhibition of STAT5a may serve as a potential new target for CRC treatment.