The purpose of the paper is to study the performance of the CANDU (Canada Deuterium Uranium) reactor when the reactor core is loaded with thorium fuel mixed with plutonium isotopes with ratio 3 and 5%. A three dimensional model is designed for the core of CANDU reactor. The computer code MCNPX (Monte Carlo N-Particle Transport) is used to calculate the processes in its core. The results are compared with natural UO 2 case which is the typical fuel of the reactor. The results show that the multiplication factor of the reactor is higher even in the case of thorium fuel mixed with 3% plutonium isotopes, which indicates longer neutron life cycle length and more economic utilization of the reactor.