Considering the relationship between the environmental load and the environmental quality is very important in design of the ecological housing. Moreover, the importance of study from the viewpoint of lifecycle is increasing. This study aims to clarify relations between LCCO 2 and thermal environment in passive solar house with thermal mass. For the case study, twelve living room models with variations of window and thermal mass are set to calculate LCCO 2 and validate thermal environment. The result indicates the characteristics of "trade-off" relations not only between embodied CO 2 and operating CO 2 , but also between LCCO 2 and thermal environment.