In order to evaluate the possibility of recycling waste in contact with environmental media, leaching tests and ecotoxicity tests were performed using samples of coal ash and slag wastes. The hazardous characteristics of the waste materials and leachates from those materials were assessed based on their concentrations of chemical components, including heavy metals. The acute toxicity tests were performed with Daphnia magna. In the ecotoxicity tests, the pH ranged from 10.87 ~ 11.11, which is too high for good Daphnia growth. Results of the acute toxicity tests showed that the coal ash as TU is 1.66 and the slag was 0.3, which was lower than the regulatory limit of 2 in both samples. Despite these encouraging results, it may be necessary to consider using the chronic toxicity leaching test because the wastes are exposed to the environment for an extended time period relative to the fifteen days used in this study.