Abstract: In the paper, with S_n method We calculated the numerical value of criticalcondition,critical radius and masses, flux distribution and neutron current densityof leakage for bare sphere and combined sphere of U~(235), Pu~(239) and U~(239). In addi-tion, We also studied the effects of parameter change, for instance, concentration.compressibility, and thickness of inert layer. The main results obtained are inagreement with the published experiment data.