Numerous proteins, peptides, and chemical agents in the venom of venomous marine animals are potentially useful biologically active molecules with pharmacological properties. The main goal of this research was to study the cytotoxic effects isolated crude venom from the Persian Gulf sea anemone, Stichodactyla haddoni against Breast, and Human embryonic kidney cell lines in vitro through using MTT assay. Samples of S. haddoni were collected from coastal waters of Lark island Persian Gulf the south of Iran. The extraction of Aqueous from tentacle was performed. The cell line was cultured in complete tissue culture medium. These cells were exposing in presence of different Serial dilution 100 to 0.78 μg of venom extract in duration time 24 hour. Analysis of statistical for cytotoxic activity of crude venom on cell lines this study showed that activity was similar together in almost all doses(Pvalue <0. 05). IC50 of crude venom against breast, and normal cell lines was observed at 4.13, 117.8µg respectively. With increasing extract concentration, death of cells on cells increased.The extraction induced more lethality on breast cell line than normal cell lines. Therefore, crude venom S.haddoni can be considered as a strong preventive grow agent against cancer cell lines especially breast cancer and with low or without toxicity on human normal cells