Quenching and partitioning (Q&P) treatment was applied to 12%Cr low-carbon martensitic stainless steels containing different amount of silicon, and then the effect of silicon content on the formation of retained austenite was discussed in terms of the behaviors of phase transformation and carbide precipitation during the partitioning treatment. It was suggested that the nearly ideal constrained carbon equilibrium (CCE) condition was realized in the specimens used in this study because a large amount of chromium suppressed bainitic transformation during the partitioning treatment, although cementite was partly observed at the retained austenite/martensite interface. The cementite precipitation was greatly retarded with increasing silicon content, resulting in an increase in the amount of retained austenite and an enhancement of the thermal stability of austenite. Tensile testing for the Q&P-treated specimens revealed that the increment of silicon content leads to a marked improvement of strength-ductility balance of the 12%Cr steels through the increase of retained austenite.