Micro-crack initiation and growth behavior were observed on the surface of nondegraded and degraded CrMoV cast steels under various strain wave shape loadings at a high temperature. The degraded steel was taken from the inside of a main steam valve casing after 140 000 h of service. No significant difference in crack growth behavior was observed in low cycle fatigue in both steels ; however, the crack growth rate in the degraded steel was higher than that in the nondegraded one in creep fatigue under S-F straining. This was caused mainly by coarsened grain boundary carbides. A residual life assessment method, based on micro-crack growth and the statistical properties of micro-crack length distribution, gives an estimation close to the experimental results.