This study develops a new type of eco-friendly building brick so-called unfired four-hole hollow brick (UFHB) by using a mixture of ordinary Portland cement (OPC), low-calcium fly ash (FA), and different proportions of crushed sand (CS) and river sand (RS). A hydraulic-static pressure was applied to form the brick samples. The effect of various CS-RS blends on the mechanical strength of the UFHB samples was studied. In addition, the numerical method was applied to simulate the heat transfer process through the brick wall. The obtained results show that the UFHB mixture containing a mixture of 10% RS and 90% CS as blended fine aggregate registered the highest strength value in comparison with other UFHB mixtures. Moreover, utilization of the UFHB was highly effective in term of heat insulation as compared to the conventional brick.