A preliminary design study is performed for small molten-salt power reactors fueled with ²³³U. Nuclear characteristics of the reactor are analyzed by using two-dimensional diffusion and burn-up calculation code "CITATION" together with 9 group reactor constants. Two semioptimized reactors (initial cores) are found out after performing the design studies of 57 reactorcores which are different from each other in the fuel salt composition, the graphite volume ratio, the geometry and size etc.. Burn-up characteristics are examined on the semi-optimized reactors for 1, 000 days after beginning the reactor operation. It is found that the fuel conversion ratios of the reactors range of 0.98-1.00 without any continuous chemical processing except the fission gas removal. The reactor can be operated for about 25 years without any exchange of the core graphite, owing to the low neutron flux density. These results support that the small molten-salt power reactor has several advantages in the aspects of operation and maintenance, safety, nuclear proliferation resistance, energy resource problems etc..