Background: Cancer refers to the abnormal growth of cells and is still the most common cause of morbidity in world. The purpose of this study was to determine cytotoxicity effect of high molecular weight (HMWC) and low molecular weight of chitosan (LMWC) on three cancerous cell lines MCF-7, HeLa and Saos-2 with different histological origin. Methods: The anticancer property of two types of chitosan on three cancerous cell lines and human fibroblast as normal cell was evaluated by cytotoxic activity and apoptosis induction .The cells were treated by different concentration of chitosan and viability was determined by MTT assay after 24, 48 and 72 h .Mode of death was determined by Annexin V staining assay for apoptosis and analyzed by flow cytometry. Results: While both types of chitosan were more efficient in inhibiting cell proliferation of three cancerous cell lines, fibroblast cells showed somehow more compatibility with chitosan .Viability of cells was reduced concentration-dependently to 70-90 of the untreated cells as control. There were no significant differences between the effect of both types of chitosan on all cell lines. Flow cytometry analysis showed necrosis more observable with MCF7 while the apoptosis pattern of death was more in Saos-2 and HeLa. Also higher viability with both types of chitosan was seen in fibroblast as normal cells. Conclusion: While chitosan is compatible with normal diploid fibroblast cells, it shows anticancerous effect against 3 cancerous cell lines. Furthermore, it seems that the molecular weight of chitosan does not affect its anticancerous property
