A Monte-Carlo parametric study was carried out to investigate some nuclear properties of LR-0 reactor when it is moderated by mixtures of light and heavy water at molecular ratios ranging from 0% up to 100% D 2 O at increments of +10% in D 2 O. Neutron energy spectrum, fission rates, non-fission neutron capture rates, neutron absorption in poison material, and average fission energy yield from 235 U and 238 U components of the UO 2 fuel were tallied at different regions in the reactor at the 11 heavy water percentage moderators being studied. It was found that the changes in all the tallied parameters with heavy water percentage in moderator were not monotonic, and the nuclear properties of the reactor changed dramatically between 90% and 100% heavy water moderators. The most prominent changes were in neutron energy spectrum, neutrons leaking out from the reactor, neutrons absorption in moderator, and non-fission neutron capture in 238 U. These changes were not the same in all regions of the reactor.