Precast concrete produced in the industry is advantageous in a sense that it meets certain requiring standards and thus is easy to manage, and it saves construction period by shortening concrete curing time in the field. Nevertheless, studies on the strength evaluation of PC material by steam curing have rarely been done. In addition，as concrete becomes of high strength, it is speculated that relevant steam curing temperature history is also required. Therefore this study is on the steam curing method in manufacturing precast concrete products, and cement mortar has been used for experiments to exclude the possibility that concrete aggregate granularity and aggregate shape change may affect on strength development by cement hydration. In addition, this research is to provide the fundamental information of industrial manufacture of PC member by suggesting the optimal steam curing condition. The optimal steam curing condition has been investigated from the relations between temperature history condition and strength development, via modifying temperature patterns in various ways such as pre-tirne, curing maximaI temperature, maximaI temperature maintenance time which are factors that affect on high strength concrete product in steam curing.