Fatigue limits of cast Ti-6Al-7Nb and Ti-6Al-4V, which have been subjected to hot-isostatic-pressing (HIP) treatment after casting (as-HIP'ed cast alloys), conducted with thermochemical treatments, which are usual thermochemical processing (THP) and THP combined with a post treatment (THPH), improve about 40% as compared with those of as-HIP'ed cast alloys. Fatigue crack initiation and small fatigue crack propagation resistance of HIP'ed cast Ti-6AI-7Nb and Ti-6Al-4V conducted with THP and THPH improve remarkably as compared with those of both as-HIP'ed cast alloys because the effective slip length under the cyclic stress decreases due to refined microstructure by THP and THPH, and then the slips occur uniformly. Fatigue crack initiation and small fatigue crack propagation lives of HIP'ed cast Ti-6AI-7Nb and Ti-6Al-4V conducted with THP and THPH are around 80similar to90% of the total fatigue lives. They increase twice as much as those of both as-HIP'ed cast alloys (i.e. around 40%).