Continuous attempts and studies have been conducted to discover a new agent that is highly effective against cancer cell with fewer side effects. One of these important new sources is marine organisms. A promising marine resource, reported in Chinese pharmacopeia as having antitumor properties, is the oyster shell. This research was designed to evaluate the cytotoxicity effect of oyster shell extract against three different cancer cells, first a sterile, 0.22 ?M syringe filter was used to filter 1000 mg of oyster shell dissolved in dimethyl sulfoxide. The stock extract was stored at -80°C and then the active ingredients were identified using liquid chromatography-mass spectroscopy (LC-MS), while the anti-proliferative activity of oyster shell extract was evaluated by 3-(4, 5- dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide assay. The analysis of oyster shell extract by LC-MS confirmed the presence of many active compounds such as coumarin, unsaturated fatty acids and glycosides. The marine oyster demonstrated significant cytotoxic activity against prostate cancer PC3 cells, with an IC50 value of 284 ?g/mL. It exhibited modest cytotoxic activity against lung cancer cells (A549) and Abelson murine leukemia cells in mice, respectively. The detected cytotoxicity of oyster extract against various cancer cell lines may open the door for future research on cytotoxic agents for cancer control.