This paper describes the effect of heat aging on thermal and mechanical properties of shape memory alloy. The materials used in this study are Ti-Ni-(6-12 at%)Nb shape memory alloy. The tensile tests at 173 K and the heating-cooling tests under constrained strain condition were carried out using the specimens aged at 561 K for between 20 h and 1000 h in normal water chemistry. The martensite start temperature rapidly decreases in the initial short aging time, but there is little further decrease of the martensite start temperature with aging time more than 100 h. The recovery stress gradually increases with aging time, but on the contrary, the aging time of more than 500 h causes reduction of the recovery stress. (doi:10.2320/matertrans.48.439) In order to develop an SMA joint with the prescribed performance, it is necessary to optimize the manufacturing conditions such as the chemical composition, the processing condition and the heat treatment condition of shape memory alloys, that is, the SMA joint should keep up the prescribed function at the storage temperature and reproduce the prescribed function in the normal usage environment. The effects of the Ti/Ni ratio, the Nb concentration, the heat treatment temperature and the processing rate on the mechanical and functional properties were discussed, and the manufacturing conditions of the SMA joint were clarified. The thermal and mechanical properties such as the pre-strain, the recovery strain, the transformation temperature and the recovery stress were also clarified so as to determine the amount of tube expansion of the joint ring. The SMA joint rings are normally used in high-temper- ature water for long-time. In such cases, there is a possibility of the functional property change due to aging. Therefore, in order to maintain the long-term stability of the function of the SMA joint ring and secure the function of the SMA joint ring at lowered environmental temperature, it is necessary to investigate the shape memory property, such as the marten- site start temperature and the recovery stress, of the SMA joint ring exposed to high-temperature water for a long term. In this study, we investigated the thermal and mechanical properties of the joint ring material exposed to high-temper- ature water for a long term, and clarified the effect of the aging time on the function property of the joint ring material, in order to evaluate the performance and the stability of the joint ring in the usage environment of the actual equipment.