To improve nuclear fuel utilization efficiency and prolong fuel cycle bu-up, a tight pitch lattice pressured heavy water reactor was investigated as an alteative of next generation of power reactors. It is shown that the high conversion ratio and negative coolant void reactivity coefficient are challenges in the reactor core physics designs. Various techniques were proposed to solve these problems. In this work, a tight pitch lattice and mixed fuel assemblies pressured heavy water reactor concept was investigated. By utilizing numerical simulation technique, it is demonstrated that reactor core mixed with Pu/U and Th/U assemblies can achieve high conversion ratio (0.98), long bu-up (60 GWD/t) and negative void reactivity coefficients.