In case of enhancing operation temperature of land base gas turbines, it is very important to understand the materials degradation mechanisms of hotgas path components. In this study, long term agings at the high temperature were conducted for three kinds of Ni-based superalloys, Hastelloy X, Nimonic 263 and 1N738LC. Through the microstructure observations and the identifications of the precipitates in these three alloys aged at various conditions, new Time-Temperature -Precipitation diagrams were determined for the alloys. The quantitative analyses of intragranular precipitates, such as volume fraction, size and inter-particle spacing of precipitates, were conducted. The degradation behaviors of these alloys aged were evaluated by the creep tests at relatively higher stress conditions. The ratio of the minimum creep rate to that of the initial as-heat-treated one was used as an indication of degradation and the ratio is called a degradation ratio in this study.