A pilot study was conducted on the alkali-silica reactivity to evaluate the effect of coal gasification slag fine aggregate (CGS), which contains a high amount of alkali on alkali-silica reactivity of concrete made with highly alkali-reactive coarse aggregate and high alkali content CGS. The results showed that in concrete prism tests with NaOH added at a total alkali content of 3.0kg/m3, the specimen containing high alkali content CGS showed ASR expansion at an early age, and the amount of expansion exceeded that of the concrete without CGS until one year of age. Polarized light microscopy of mortar specimens made by wet screening from fresh concrete used in the concrete prism test confirmed that the high alkali content CGS itself did not cause an alkali-silica reaction, although the outer edge of the CGS seems to have little reaction. In the pore solution analysis of the mortar specimens, the Na, SO42- and OH- concentrations tended to increase with the mix of high alkali content CGS. In conclusion, alkaline leaching from the coal gasification slag fine aggregate, which contains a high alkali, induced an alkali-silica reaction in the highly alkali-reactive aggregate.