Objective: To determine the lead bioactive compound in kernel extract of Mangifera pajang and its anti-cancer activity against human breast cancer cell lines with positive estrogen receptor (MCF-7). Methods: The methanolic extract of dried powder kernel of Mangifera pajang was exposed to column chromatography for isolation. The structural elucidation of the isolated compound was characterized using infrared, nuclear magnetic resonance, mass spectrometry. Furthermore, cytotoxicity, morphological changes, flow cytometry and cell cycle arrest analyses were performed to examine the mechanism of anti-proliferation and apoptosis induced by methyl gallate against MCF-7. Results: One compound was isolated from the methanolic extract of Mangifera pajang kernel and identified as methyl gallate. The flow cytometric results demonstrated induction of apoptosis in MCF-7 cells by three concentrations of methyl gallate. The cell cycle arrest showed a significant (P<0.05) decrease in cell progression at G 2/M phase of MCF-7 after treatment with 100 μM of methyl gallate. The cell percentage of early and late apoptosis was significant at 10 and 100 μM of methyl gallate. Also, methyl gallate treatment induced up-regulation of reactive oxygen species levels in MCF-7 cells with a reduction in superoxide dismutase levels. Conclusions: These findings indicate that isolated methyl gallate from Mangifera pajang kernel extracts induces growth inhibition and apoptosis in MCF-7 cells via up-regulating oxidative stress pathway.