Despite an improvement in the efficacy of chemotherapeutic agents, the outcome of patients with prostate cancer remains poor. MicroRNA (miRNA/miR)-139 expression is often downregulated in multiple types of tumor, including in prostate cancer. The aim of the present study was to investigate the inhibitory effect of miR-139 on the PC-3, C4-2B and LNCaP prostate cancer cell lines. Analysis of the cell cycle of PC-3, C4-2B and LNCaP cells transfected with miR-139 revealed a significantly increased percentage of cells in the G(1) phase and a decreased percentage in the S and G(2) phases compared with those transfected with a negative control miRNA. The growth inhibitory rate of miR-139-transfected cells 24, 48 and 72 h after transfection were 32.83 +/- 2.61, 52.58 +/- 3.2 and 62.36 +/- 4.55% in PC-3 cells; 30.28 +/- 2.25, 51.74 +/- 3.27 and 60.80 +/- 3.58% in C4-2B cells; and 33.20 +/- 2.67, 51.83 +/- 3.59 and 61.79 +/- 4.85% in LNCaP cells, respectively. The present study revealed that miR-139 inhibited the proliferation of prostate cancer cells by interfering with the cell cycle. Further study into the mechanism by which this happened suggested that miR-139 reduced cyclin D1 expression and inhibited cell proliferation through targeting Notch1.