CANDU reactor flexibility in using advanced fuel options allowed the continuous improvement of its performances. Both economic benefits and better operational safety are expected from thorium-based fuel alternatives. This paper addresses the use of uranium – thorium mixed oxide in 37- and 43-element CANDU fuel bundles, in the aim of obtaining better economic and safety performances. Thorium is expected to partially replace uranium in the fuel, the expected Th/U ratio being 1.5 ÷ 2.333. These bundles are meant to be loaded in every channel of the CANDU core, in a homogenous “once-through Thorium” (OTT) core. The low-enriched uranium (LEU) with 5 ÷10 wt% U-235 is a suitable fissile driver material for thorium in CANDU, allowing the chain reaction to be initiated throughout the core. Cell calculations were performed using the transport codes DRAGON and WIMS, as well as the associated nuclear data library, in order to estimate the key fuel performance parameters: maximum burnup, power peaking factors, fissile utilization, void reactivity, Doppler effect and proliferation resistance.