Thermal resistance of the external walls of buildings is highly important for reducing and optimizing the energy consumption needed for heating and cooling the buildings. Identifying and evaluation of the thermal performance of common external wall systems can be very effective in improving the heat resistance. The hollow clay- block system extremely are in use in the Iran construction system, then optimizing the heat resistant of them could be highly effective on energy consumption at building part in Iran . In this paper, the hollow clay-block systems, which are commonly used in the world and especially in Iran, have been investigated in thermal performance. At first, the researches on the shape of the clay blocks that evaluate the heat transfer rate have been mentioned. Then the effects of each of the three modes of heat transfer, namely conduction, convection and radiation are investigated in The 3 most common types of clay blocks used in the external wall systems in Iran. They have been simulated in FlUENT software and the quality and quantity of thermal performances of them are extracted and compared with the results. All of the effective parameter in the modeling and calculating process at the 3 models were supposed as same. Thus the result could indicate the important effect of the geometry of the blocks on their heat resistance. Finally, the analysis results propose optimal solutions for the design of clay blocks. Increasing the pass of the heat transfer from high temperature side to low temperature side, decreasing the internal partitions that directly go through the heat transfer vector; they work as a bypass, increasing the internal partition that cut through the heat transfer vector, they prevent from radiation heat transfer and reduce perimeter of the hollows; it lessen velocity magnitude of convection in the hollows. Not using rectangular profile for hollows help to reducing heat transfer through radiation. Using insulation in clay blocks changes the percent of heat transfer from conduction, convection and radiation; it grows up the conduction rate and reduces convection and radiation and finally increases the thermal resistance. The material that sticks hollow clay blocks together also works as a bypass, thus if it were removed from the wall system; the thermal behavior and resistance became highly optimized. All of the shape optimizing process could rises the weight of the hollow clay block and it is a negative factor. The weight of hollow clay blocks and its thickness are two of most important parameter that effect on thermal behavior of hollow clay blocks. The material and its parameter as density and radiation factor are also highly effective on thermal resistance, but in this paper they weren’t involved because we want to study the shape and geometry of the hollow clay block and to reach some proposes to optimize and improve their thermal resistance. The proposals if combine with a newer system of assembling of wall component will achieve better results.