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 ²³³U and ²³⁵U isotopes have been used as fissile element of thorium oxide fuel. ¹³⁵Xe and ¹⁴⁹Sm reactivity variations has been studied in the core loaded (Th- ²³³U)O₂ or (Th- ²³⁵U)O₂ fuel matrixes during 3 months burn up process. Results: Thorium oxide having 4% ²³³U burned 1 MW power results in less ¹⁴⁹Sm reactivity than thorium oxide having 4% ²³⁵U burned in 0.5 MW power. ¹³⁵Xe 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 ²³⁵U burns into the core using 0.5 MW power and 364 g of ²³³U invents after 3 months. Burn up calculation of the modeled core of (Th-²³³U)O₂ 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 this time and then neutron multiplication factor is steadier up 3 months. Conclusion: Breeder thorium fuel enriched ²³³U make several advantages of good neutronic economy, ²³³U inventory and less inventory of long-lived alpha emitter wastes.