Increasing the size of type 13 Cr-Ni martensitic stainless steel castings with complicated cross sections results in difficulty in obtaining a uniform temperature distribution during thermal treatment, which in turn leads to a critical condition for cracking, in addition to deterioration of impact properties due to slow cooling. Chromium carbides (Cr₂₃C₆) precipitated by slow cooling through the austenite region at prior austenite or δ-ferrite grain boundaries cause an adverse effect on toughness, giving mosaic-like intergranular failure. Reducing silicon and carbon, along with the addition of molybdenum, improves toughness as the result of suppressing the Cr₂₃C₆ precipitate. Toughness degradation due to slow cooling from the tempering temperature was considered to be associated with the decomposition of interlath-retained austenite and the solubility change of carbon. Lowering the silicon content and the addition of molybdenum lessen the toughness deterioration.