Introduction: Cervical cancer is a leading cause of death in women. Current cancer treatment comes with side effects. Clinacanthus nutans has been known traditionally to treat cancer. This study was aimed to characterize C. nutans standardized fraction (SF1) and to investigate its anticancer mechanism against SiHa cells. Materials and Methods: SF1 was produced by optimized methodology for bioassay-guided fractionation. Fourier transform infrared (FTIR) spectroscopy and liquid chromatography-mass spectrometry (LC-MS) were carried out to characterize the SF1. SF1 was screened for cytotoxicity activity toward HeLa, SiHa, and normal cells (NIH) cells by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay. The anticancer mechanism of SF1 was evaluated toward SiHa cells, which showed highest cytotoxicity toward SF1 treatment. The mechanism includes cell cycle progression and protein expression, which was detected using specific antibody-conjugated fluorescent dye, p53-FITC, by flow cytometry. Results: Major constituents of SF1 were alkaloids with amines as functional group. SF1 showed highest cytotoxic activity against SiHa (half-maximal inhibitory concentration [IC50] < 10 mu g/mL) compared to HeLa cells. Cytoselectivity of SF1 was observed with no IC50 detected on normal NIH cells. On flow cytometry analysis, SF1 was able to induce apoptosis on SiHa cells by arresting cell cycle at G1/S and upregulation of p53 protein. Conclusion: SF1 showed anticancer activity by inducing apoptosis through arrested G1/S cell cycle checkpoint-mediated mitochondrial pathway.