Information about the leachability of radionuclides contained in cement-waste composites is necessary to assess their safety for storage for a period long enough to allow the radioactivity to decay to a negligible level and final disposal. To assess the safety for disposal of a radioactive waste-cement composite, the leaching of 137Cs from a waste composite into a surrounding fluid was studied. Leaching tests were carried out in accordance with a method recommended by IAEA [International Atomic Energy Agency]. The leachability was measured as functions of waste to cement ratio (Wa/C), concentration of salt (Na2SO4), temperature of leachant and curing time of specimens. The fraction of 137Cs leached from a specimen of Portland cement is 0.3-0.4 at the leaching time of 100 days, the corresponding value for a specimen of slag cement is 0.1-0.15. The leachability depends on the structure of cement composite which is influenced by such factors as Wa/C, salt concentration of the wastes, and curing time of the specimens. The leach coefficient of 137Cs increases with increasing porosity of the cement composite. At lower temperature, curing time of the specimens significantly affects the leachability, probably due to insufficient curing. A curing time of approximately 50 days before the leaching test is enough to eliminate the apparent effect of temperature.