For the preliminary judgement on the chemical stability of concrete aggregates mixed with cement paste, ASTM C 295 method can be applied prior to the long-term chemical test methods. By using this standard test method, the petrographic study on the appropriateness of natural KEDO aggregates for concrete was carried out. With the natural gravel and sand aggregates, the polarized microscope, stereoscopic microscope, and X-ray diffractometer were used for examination. The result shows the 23% of gravel aggregates and 5.1% of sand aggregates are chemically unstable. To select the favorable KEDO concrete aggregates, it is required to exclude the highly metamorphosed rocks, acidic volcanic rocks, highly foliated rocks, and expansive rocks identified from mortar-bar test. Further chemical test and mortar-bar test method integrated with this study is recommended for the suitability assessment of natural KEDO concrete aggregates.