Abstract: Objective To investigate the relationship between expression of p21-actived kinase 5（PAK5）in osteosarcoma tissue and sensitivity of neoadjuvant chemotherapy，and whether the activation of PAK5 is involved in regulating the sensitivity of osteosarcoma cells to chemotherapy at cellular level. Methods PAK5 expression in biopsy tissues of 74 osteosarcoma patients were detected by immunohistochemistry.The relationship between PAK5 expression and clinicopathological features of osteosarcoma was analyzed. siRNA-PAK5 was design and synthezed，and it was transferred into Saos-2 and MG63 cells. PAK5 mRNA and protein was detected by QPCR and Westem blotting. Saos-2 and MG63 cell were treated with different concentrations of adriamycin（ADM 50, 25, 5, 0.5 μmol/L） and methotrexate（MTX 50, 5, 1, 0.5, 0.05 μmol/L） respectively. Cell proliferation inhibtion rate was detected by CCK-8 assay. The difference of half inhibiton concentration （IC50） of osteosarcoma cells after transfection of siRNA-PAK5 was analyzed. Results In total of 74 osteosarcomatissues， PAK5 was positive in 57 case.The expression of PAK5 was related to the tumor necrosis rate after neoadjuvant chemotherapy and lung metastasis.The levels of PAK5 mRNA in the transfected group were decreased to 29％ and 31％ in the non transfection group， respectively；Western blotting showed that the expression of PAK5 protein in transfected group was lower than that in non transfected group by 49％ and 42％. After transfeced with siRNA-PAK5， the IC50 of ADM in Saos-2 and MG63 were decreased from（8.35±0.07）μmol／L，（7.35±0.07）μmol／L to（0.54±0.004）μmol／L，（0.50±0.002）μmol／L， respectively；the IC50 of MTX in Saos-2 and MG63 were decreased from（1.255±0.021）μmol／L，（1.05±0.014）μmol／L to（0.606±0.01）μmol／L，（0.72±0.014）μmol／L，respectively. Conclusion The expression of PAK5 in osteosarcoma is related to the sensitivity of neoadjuvant chemotherapy， inhibiting the expression of PAK5 in osteosarcoma cells， which can improve the sensitivity of chemotherapy.