The aim of this study was to investigate the effect of a small interfering RNA (siRNA) targeting human epidermal growth factor receptor 2 (HER2/neu) on the proliferation and viability of prostate cancer PC-3M cells. Chemically synthesized siRNA targeting HER2/neu was transfected into PC-3M cells by using liposomes, and cells transfected with empty liposomes, a negative siRNA sequence, or nothing (untransfected) were used as controls. mRNA and protein levels of HER2/neu were detected using reverse transcription-polymerase chain reaction and western blot, respectively. The inhibitory action of HER2/neu siRNA on the in vitro growth of PC-3M cells was assessed by the cholecystokinin 8 assay and apoptosis was detected using flow cytometry. Cells transfected with HER2/neu siRNA showed decreased mRNA and protein levels of HER2/neu compared to control groups (P < 0.05). The survival rate of PC-3M cells decreased significantly after transfection with HER2/neu siRNA compared to that of untransfected cells (55.39 ± 1.60 and 81.42 ± 0.80%, respectively; P < 0.05). The apoptosis rate in cells transfected with HER2/neu siRNA was quite high (45.60 ± 0.70%) compared to that of blank control, empty liposome, and negative siRNA sequence groups (P < 0.05). In conclusion, siRNA targeting HER2/neu inhibits HER2/neu expression in PC-3M cells, resulting in an inhibition in proliferation and an induction of apoptosis.