Abstract: ObjectiveTo study the changes of expression of CDKN1A gene (p21) after the X-ray radiation on breast cancer cell line (MCF-7) and the effect of expression of CDKN1A on cellular apoptosis. MethodsExpression of CDKN1A gene in MCF-7 cells was detected at different time and radiation dose by flow cytometric test. Cellular apoptosis was analyzed under CDKN1A gene inhibition by RNA interfference (RNAi). ResultsAfter the X-ray irradiation on MCF-7 cells with 1，2 and 4 Gy, the expression of CDKN1A protein increased gradually, in which 4 Gy irradiation obtained the maxiamal expression（P<0.05）. With 4 Gy radiation，the expression level of CDKN1A protein improved differently after 8, 12, 24, 48,72 h and resulted in triple increase (P<0.05) after 24 h of radiation. And the inhibition of CDKN1A expression with RNAi, compared with control group, the apoptosis rate of MCF-7 cells increased 183.9% (P<0.05). ConclusionAfter 4 Gy radiation, the expression of CDKN1A of breast cancer cells reached the peak at 24 h and the inhibition of its expression might promote the cellular apoptosis.