Effects of MC and M₂₃C₆ carbides on the creep rupture strength of 18%Cr-10%Ni-Ti-Nb stainless steel were quantitatively evaluated. The solubility relationship for (Ti, Nb) C was determined, as creep rupture strength depends on the amount of carbon dissolved during solution treatment. On one heat, the solution treatments at various temperature were conducted and the quantitative relationship between MC carbide and creep rupture strength was obtained by carrying out creep rupture tests at 650°C. On the basis of this result, the effect of M₂₃C₆ carbide on the creep rupture strength was calculated by deducting the contribution of MC carbide from the practical rupture strength. The following formula was obtained for estimating the rupture strength of this steel. (650°C, 10000h creep rupture strength) =21.5×(C as MC)+6×(C as M₂₃C₆)+8.5(kgf/mm²)