miRNA was 20～25 nt endogenous non-coding RNA. miRNAs are encoded small RNAs that hybridize with messenger RNAs, resulting in degradation or translational inhibition of targeted transcripts. In order to investigate the function of miR-138 on mouse mammary epithelial cells, technique for gene silencing-miRNA inhibitor (anti-miRNA) was applied to make miR-138 silence, qRT-PCR was showed valid for inhibitor miR-138. And Western blot, CASY^®-technology was put in use to study some change of mouse mammary epithelial cells after miR138 inhibitor. It was shown that miR-138 suppresses the exepress of PRL-R(P < 0.05). It was proved that miR-138 can inhibit activity and proliferation of mouse mammary epithelial cells after miR-138 inhibitor(P < 0.01). HPLC was used to study expression of β-casin mouse mammary epithelial cells after miR-138 inhibitor, it shows down express of β-casin(P < 0.05). Results indicated that the target of miR-138 was PRL-R, miR-138 suppresses translation of PRL-R. MiR-138 suppressed the viable and proliferation of mouse mammary epithelial cells.