Objective:Studies showed that Newcastle disease virus (NDV) had significant inhibitory action on many tumors in vitro and in vivo. This experiment was designed to investigate the effects and possible mechanism of NDV－D90 strain on breast cancer MCF－7 cells in vitro. Methods:Growth inhibition of MCF－7 cells was detected by CCK－8 method. The morphological changes of MCF－7 cells caused by D90 strain were observed by optical microscope and DAPI staining. MCF－7 cell apoptosis rate was detected by flow cytometry. The expression of apoptosis related proteins of MCF－7 cells was detected by Western blot. Results:NDV－D90 strains had inhibitory effect on MCF－7 cells in a concentration and time dependent manner;D90 strain induced apoptosis of MCF－7 cells and regulated the expression of apoptosis related proteins,including up－regulating Bax expression,down－regulating Bcl－2 protein,and promote the cleavage and activation of Caspase－3 and Caspase－9. Conclusion:D90 strain could regulate the expression of apoptosis related protein,promote the cleavage and activation of Caspase－3 and Caspase－9. It could induce apoptosis of MCF－7 cells through the mitochondrial pathway in a concentration and time dependent manner.