The cytotoxic effect towards myoblast cancer cells (C3C12) of well-crystalline colloidal Pt quantum dots (QDs) was examined and presented in this paper. The Pt QDs were synthesized by facile colloidal process and characterized by transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM), which confirmed that the QDs are prepared in high-density, possessing a well-crystalline structure. To examine the cytotoxicity, various doses of as-prepared QDs were treated with C2C12 myoblast cancer cells at different incubation intervals; 24, 48, 72, and 96 hours, and the viability of cells was analyzed with MTT assay. The detailed analyses revealed that in the presence of Pt-QDs, with increasing the incubation time, the number of cancer cells decreases. Moreover, with increasing concentration of Pt-QDs, the cancer cell death increases, confirming that the concentration of Pt-QDs has a significant role in controlling the number of cancer cells. The present research demonstrated that Pt-QDs can efficiently be used as anticancer agents.