Background: Breast cancer is the second leading cause of cancer-related death worldwide among women. Nowadays, nanoparticles are interested as anti-cancer factors in studies related to cancer therapy. Therefore, the present study aimed to explore cytotoxic effects of zinc oxide nanoparticles (ZnONPs) and to evaluate NM23 gene expression in T47D breast cancer cells. Materials and Methods: The cancerous T47D and normal HEK293 cell lines were treated with different concentration of ZnONPs for 24 hours and the cytotoxicity activity of ZnONPs was analyzed by the MTT assay. Then, the NM23 gene expression was evaluated using the real-time PCR method. Results: According to MTT results, the ZnONPs significantly decreased the viability of T47D cells in dose-dependent manner. Also, the results revealed that the mRNA level of NM23 was up-regulated (7.8-fold) in cells treated with ZnONPs. Conclusion: ZnONPs can destroy cancerous T47D cells compared to normal HEK293 cells. Furthermore, it seems that ZnONPs can modulate metastasis by enhancing the NM23 gene expression level in breast cancer T47D cells. Thus, ZnONPs can be considered as a promising strategy for the treatment of breast cancer