Background: Recently, more attention has been paid to the use of complementary medicine in breast cancer (BC) treatment.Juglans green fruit pericarp was found to have anti-cancer characteristics. Objectives: We aimed here to assess the possible anti-proliferative effects of methanolic extract of Juglans green fruit pericarpin BC cell lines.Methods: MCF-7 and MDA-MB-231 cells were examined with different concentrations of Juglans green fruit pericarp extract. Thecell viability was investigated using a 3 - 2, 5-diphenyl tetrazolium bromide (MTT) assay. Also, different phases of the cell cyclewere analyzed utilizing a flow cytometry method. Finally, the ability of cancer cells to create colonies was studied post-treatment with Juglans green fruit pericarp.Results: Our findings showed that the methanol extract has substantial cytotoxicity on MCF-7 and MDA-MB-231 cell lines. Wealso observed a significant arrest in the G0/G1 phase in cancer cells post-treatment with the methanol extract.Conclusions: In summary, our results introduced a cytotoxic property for Juglans green fruit pericarp methanol extract for MCF-7 and MDA-MB-231 cell lines. Future studies are needed to further evaluate its utility as a potential complementary therapy for BC
