Abstract: TOPAZ-Ⅱ which is fueled with highly enriched uranium and moderated with zirconium hydride is a reactor for space electric power supply. But difference with normal reactors moderated by ZrH, moderator temperature effect of TOPAZ-Ⅱ reactor is positive, and the value is large. The moderator temperature effect of TOPAZ-Ⅱ reactor was calculated with MCNP in this paper. The reason why moderator temperature effect is positive was got by analyzing the changes of neutron spectra of main regions, neutron production rates, neutron absorption rates and leakage rates at different moderator temperature. When moderator temperature rises, neutron production rate large increases and leakage slight increases, but neutron absorption rate decreases, so TOPAZ-Ⅱ reactor presents positive moderator temperature effect.