Microcracks developed considerably in concrete subjected to elevated temperature up to around 60°C at early ages, especially in low water-to-binder ratio (0.3) concrete. Microcracking was attributed to the stresses induced by the incompatibility in deformation between mortar and aggregate. Differences of coefficients of thermal expansion (CTE) between mortar and coarse aggregate, autogenous shrinkage of mortar and size of coarse aggregate were important factors influencing deterioration. The tensile strength of concrete was severely affected by the extent of microcracks. Concrete using ground granulated blast furnace slag (GGBFS) suffered worse damage than concrete prepared from ordinary Portland cement alone. Attempts were made to apply the acoustic emission (AE) technique to study the process and mechanism of microcracking. The skills required to practice the AE technique at early ages and at high temperature were carefully considered. AE hits agreed with the test results for deformation and tensile strength. Most of the microcracks occurred within the descending period of temperature and were classified into tensile mode. The use of coarse aggregate with larger CTE, saturated fine lightweight aggregate, and the reduction of the maximum size of the aggregate were greatly effective in reducing microcracking and improving the tensile strength of concrete made with GGBFS. Direct tensile strength was more adversely affected by microcracking than splitting tensile strength.