Abstract: The reactor core parameters of miniature neutron source reactor (MNSR) with high enrichment uranium (HEU), such as k_eff, β_eff, reactivity worth of the center control rod, reactivity worth of the top beryllium, inner irradiation site neutron flux density and shut down margin, were calculated by Monte-Carlo code. Comparison between the calculated and the experimented results was performed，and they were in agreement with each other very well. Without changing the core dimensions of the MNSR, but substituting the HEU fuel with low enrichment uranium (LEU) fuel and Al cladding with zircaloy cladding, the k_eff was calculated at the ²³⁵U enrichment from 12.0% to 12.5%, and the fitting enrichment of 12.5% was chosen. The reactor core parameters of MNSR with LEU of 12.5% were calculated. From the above calculated results, it is concluded that the conversion of MNSR with LEU of 12.5% is feasible.