Studies have confirmed that Period2 (PER2), a core component of circadian oscillator, plays a key role in regulating biological circadian rhythm fluctuations including protein metabolism through transcriptional effects. This study employed bovine mammary epithelial cells (BMEC) as a model to investigate the effect of the suppression of PER2 expression on synthesis of casein protein. BMEC cells were established by enzymatic digestion of mammary tissue from mid-lactation cows and cultured in specific growth medium (DFEM/F12 as basis). Second passage BMEC were divided into a treatment and a control group both of which were transiently-transfected with siRNA or a negative control oligonucletoide. After 72 h incubation to ensure the suppression of PER2 expression, the mRNA abundance and protein abundance of CSN1S1, CSN1S2, CSN2, CSN3, and PER2 were measured by qRT-PCR and ELISA. Statistical analysis of data were performed using an independent sample t-test module (SPSS16.0), with differences considered significant at P < 0.05. The results showed that PER2 silencing enhanced the synthesis of CSN1S1, CSN1S2, CSN2, and CSN3 in part through upregulating the mRNA abundance of CSN1S1 and CSN1S2. In conclusion, our results provide the basis for further studies aiming at understanding how PER2 regulates mammary gland function and milk protein synthesis.