The aim of the present study was to compare the degree of cytotoxicity of newly developed calcium phosphate-based root canal sealer with iRoot SP sealer on fibroblastic cells using MTT assay.The experimental calcium phosphate powder was mixed with 1% di sodium hydrogen phosphate liquid to obtain the set cement. iRoot SP sealer was used as a comparative material. Set materials were sterilized using ethylene oxide gas. Four grams of each material were soaked in minimum essential medium with Hanks salt for seven days, this was followed by shaking, and cold centrifugation then filtrated to obtain the extracts. Serial dilution (12 concentrations for each material) was obtained. The source of fibroblastic cells was baby hamster kidney (BHK-21) obtained from American Type Culture Collection (ATCC). Cellular viability was assessed using MTT assay. Four plates of 96 wells each were used. One plate served as a control. Each of the remaining 3 plates was divided into two 48 wells. The cells in 48 wells were incubated with extracts of the experimental sealer, and the cells in the other 48 wells were incubated with extracts of iRootSP, and then evaluated at 24 hour, 72 hours and one week. The results showed that after 24 and 72 hours, all concentrations except 6.25 mg/ml, exhibited no statistically significant difference between viability % of the two materials. Using 6.25 mg/ml, iRoot SP showed higher viability % than experimental calcium phosphate cement. After 7 days, using 25, 12.5, 3.12, 1.56, 0.39, 0.19, 0.097, 0.048 and 0.024 mg/ml; there was no statistically significant difference between viability % of the two materials. Using concentrations 50, 6.25 and 0.78 mg/ml; iRoot SP showed higher viability % than experimental calcium phosphate cement. In conclusion: the newly developed calcium phosphate sealer was considered not cytotoxic and comparable to iRoot SP. Key Words: Cytotoxicity, Calcium phosphate-based root canal sealer.