Background: Decrease of economically accessible uranium resources motivates consideration of breeding of fertile elements such as thorium. Material and Method: Thorium oxide fuel burn up calculation of a simulated research reactor cooled heavy water has been proposed in the present work using MCNPX 2.6 code. Two U-233 and U-235 isotopes have been used as fissile element of thorium oxide fuel. Xe-135 and Sm-149 reactivity variations has been studied in the core loaded (Th-U-233)O-2 or (Th-U-235)O-2 fuel matrixes during 3 months burn up process. Results: Thorium oxide having 4% U-233 burned 1 MW power results in less Sm-149 reactivity than thorium oxide having 4% U-235 burned in 0.5 MW power. Xe-135 reactivity has an overestimated shift by 15 days in the core operated in 0.5 MW than the other, after 15 days both the cores behave similarly. 480 g of U-235 burns into the core using 0.5 MW power and 364 g of U-233 invents after 3 months. Burn up calculation of the modeled core of (Th-U-233)O-2 fuel shows a fissile mass reduction by 60 days while the consumed fissile mass reaches to its initial value after 120 days. The core flux is constant during 3 months for both modeled cores. A considerable negative reactivity occurs up to 15 days in both cores which can be refer to xenon inventory during thistime and then neutron multiplication factor is steadier up 3 months. Conclusion: Breeder thorium fuel enriched U-233 make several advantages of good neutronic economy, U-233 inventory and less inventory of long-lived alpha emitter wastes.