This study aims to develop a simplified estimation method of the thermal design load and space radiant environment in order to achieve adequate and economical HVAC equipment sizing and confirmation of thermal comfort. The author proposes a new definition for the 2.5% thermal design load and corresponding operative temperature (OT). The 2.5% design load is the load which occurs at 2.5% cumulative frequency of occurrence during the summer or winter months. Hourby- hour dynamic simulation through a year is necessary to obtain the 2.5% design load and corresponding OT. The author developed a simplified estimation method for these parameters in the case of office buildings in Tokyo through simulation analysis applying the DOE technique.