Abstract: All the operating CANDU heavy water reactors are using natural uranium as the fuel, however, owing to the unique design features, the heavy water reactor actually is with high fuel flexibility, low-enriched uranium, anduranium and thorium to be reprocessed as well. The paper intends to study the potential impact of fuel changing from natural uranium to thorium fuel on core characteristics and core safety features. This paper uses DRAGON code to set up the infinite lattice model, and then calculate and compare the important core parameters for natural uranium and thorium-fueled heavy water reactor core. The study results show that the core features of thorium-fueled reactor are different from those of natural uranium-fueled reactor, but utilization of thorium will effectively help to improve the safety features of the heavy water reactor.