Abstract: Elevated levels of phosphatase and tensin homolog deleted on chromosometen (PTEN ) have been reported in mammary gland tumors in humans. This study was designed to study the expression of PTEN mRNA in canine mammary tumor tissues. We investigated 38 canine mammary gland tumors(including 15 benign mammary tumors and 23 malignant mammary tumors), 4 normal mammary gland by real-time RT-quantitative PCR- PTEN gene expression levels in canine malignant mammary tumors were much lower than that in benign mammary tumors and normal mammary tissues (P < 0. 001) ; The expression of PTEN in benign mammary tumors vary insignificantly than that in normal mammary tissues(P>0.05); The expression of PTEN in canine mammary cancer without lymph node metastasis vary insignificantly contrast to mammary cancer with lymph node metastasis(P>0.05) .The expression of PTEN was not correlated with tumor size and patient' age. In conclusion, the abnormal expression of PTEN gene protein plays an important role in the development of canine mammary tumors. It may be a predictor for biological behaviors and prognosis of breast cancer