In mass concrete constructions, cement hydration produces much heat at an early age. Because of the different distribution of temperature in their parts, which leads to a significant temperature difference between the concrete block’s center and outer surface. When the temperature difference among structural parts surpasses the permissible temperature difference, thermal cracks can occur. Hence, this article presents the development of the temperature field in mass concrete structures with two different concrete mixes M1 and M2. M1 consists of 100% ordinary Portland cement, and M2 consists of 65% ordinary Portland cement and 35% fly ash. Furthermore, by using the finite element method, the effect of the initial temperature of concrete mixtures (M1, M2) on the temperature field in the structure has been investigated. The researched results can be used to reduce the risk of thermal crack formation in mass concrete.