The M23C6 precipitation behavior in a cast Ni-base superalloy M963 was investigated after tensile creep testing at 800 degrees C and strain-controlled low cycle fatigue testing at 700 - 950 degrees C. During high temperature creep and low cycle fatigue, the primary MC decomposed into M6C continuously, and a great amount of secondary carbide, chromium-rich M23C6, precipitated preferentially in the periphery of MC and gamma + gamma' eutectic at grain boundaries. M23C6 was rarely present in grain interior, indicating that grain boundary promoted M23C6 carbide precipitation. The M23C6 precipitation was closely dependent on the stress state and testing temperature, seemed to be independent of the total strain amplitude. M23C6 is unstable during low cycle fatigue testing. The occurrence of M23C6 precipitation was sharply reduced during low cycle fatigue testing at 950 degrees C. Crack was easily initiated at interface between MC and matrix, while fine M23C6 was effective to prevent grain boundary migration. (C) 2005 Springer Science + Business Media, Inc.