Thermal, environment and economic impact of Trombe wall system for residential building in Mediterranean region will be studied in this paper. Moreover, optimum size of this system will be determined by using Life Cycle Cost (LCC) criterion. This will lead to develop an approach for designing the most economic residential building in Mediterranean region. Hour-by-hour energy computer simulations were run with TRNSYS program to analyze the performance of Trombe wall. Then, a complete economical equations of LCC have been built and optimized for variable Trombe wall area ratio as a function of economic and thermal parameters. The optimum Trombe wall area ratio from thermal and economical point of view is 37%. This optimum ratio has reduced LCC by 2.4%. Moreover, about 445 kg CO(2) will be reduced annually. (C) 2011 Elsevier Ltd. All rights reserved.