Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to examine the microscopic appearance of Alkali-Silica Reaction (ASR) products. Four typical active siliceous rocks were immerged in 1 mol/L NaOH solution at 80℃ for 7 days and 14 days. SEM photos and EDS results of ASR products were analyzed. The typical microstructure and components of ASR products were determined, and the differences between ASR products and cement hydration products in cement core sample were found out by  comparisons. The study provided scientific basis for diagnosing ASR in concrete.