Aim: The Effect of a Mineralized Bone Graft on the Surface Microhardness of Mineral Trioxide Aggregate and Biodentine Introduction: This study was designed to determine the effect of Osteon II mineralized bone powder on the surface microhardness of two retrofilling materials: Mineral trioxide aggregate (MTA) and Biodentine (BD). Methods and Materials: Each retrograde material was mixed and carried into 60 sterile custom–made Mineralized bone graft materials negatively affect surface microhardness of both MTA and BD. In presence of osteon II, BD had the highest surface microhardness.submerged in simulated tissue fluid and incubated at 37°C and 100% relative humidity for 7 days. Surface microhardness values of each study group was attained using Vicker’s microhardness test performed by UHL–VMHT microhardness tester. Conclusion: Mineralized bone graft materials negatively affect surface microhardness of both MTA and BD. In presence of osteon II, BD had the highest surface microhardness.