VHTR (very high temperature reactor) is one of Generation IV nuclear reactors. HTGR (high temperature gas cooled reactor) is a VHTR type of reactors with a graphite moderator, helium gas coolant with UO2 fuel and outlet coolant temperature of 900°C or higher than that. In this study, instead of using UO2 fuel, we will evaluate the utilization of PuO2 and ThO2 as a mixed fuel in HTGR. The burnup period is 1100 days, which corresponds to 3 years of fuel cycle length. The reactor 3D calculation was performed by using CITATION-SRAC 2006 code, with the nuclear data library was derived from JENDL3.3. The neutron spectra become harder with the boosting of the plutonium fraction in loaded fuel. Moreover, the neutron spectra at EOC become softer compared to that of BOC. Fascinatingly, U-235 is generated more with the increasing of plutonium fraction.