The plant Cynanchum sarcomedium is an underutilized, xerophytic perennial shrub that belongs to Apocynaceae. The present study aims at evaluating the effects of methanolic extract of C. sarcomedium on breast cancer and colon cancer cell lines. Cell viability was assessed by MTT assay and apoptosis was determined by morphological observation through DAPI staining. Mitochondrial membrane potential was evaluated by JC-1 staining. Cell cycle analysis and western blot analysis on the expression of proteins associated with mitochondrial cascade was also carried out. In MTT assay of MCF-7 cell lines, maximum cytotoxicity was observed at 50 µg/ml for 48 h by the methanolic extract. So, further studies have been carried out under this experimental condition. Whereas in HCT-15 cell lines, 100 µg/ml for 48 h was the effective experimental condition. Flow cytometricanalysis of treated MCF-7 and HCT-15 cell lines demonstrates that cells were arrested at the G0/G1 phase and western blot analysis indicates the activation of proteins in the mitochondrial signaling pathway. Thus, the study suggests C. sarcomedium as a promising candidate having significant anti-proliferative activity. Hence, it merits further exploration as a potential therapeutic drug.