AIM: To investigate whether RNA interference (RNAi) induced by small interference RNA (siRNA) could suppress Polo- like kinase- 1 (Plk 1 ) expression and its effects in A549 cells. METHODS: A recombinant plasmid containing siRNA targeting Plk1 ( psiRNA - hH1 - Plk1 ) was transfected into A549 cells with Lipofectamine 2000.Expressions of Plk1, cyclin B1 and p53 protein were detected by Western blotting. Cell proliferation was evaluated by direct cell counting, while cell cycle and apoptosis were examined by flow cytometry, and expression of α - tubulin was detected by immunofluorescence. RESULTS: The results demonstrated that sequence specific siRNA targeting Plk1 was capable of suppressing Plk1 expression, and reflecting in lower kinase activity in A549 cells. The level of Plk1 protein was reduced by at least 70％ after 48 h of psiRNA - hH1 - Plk1 treatment relative to controls. Expressions of cyclin B1 and p53 were increased greatly after Plk1 depletion, and cells showed absence of microtubule polymerization and spindle abnormalities in staining for α -tubulin. Growth inhibition, G2/M arrest and apoptosis were observed in psiRNA -hH1 -Plk1 transfected group. CONCLUSION: All these data suggest that siRNA targeted against human Plk1 may be a valuable tool in cancer therapy.