Two lignites and two sub-bituminous coals were treated by hot water extraction(HWE) and hydrothermal treatment(HTT), and the simulated effluents were obtained. The acute toxicity of these effluents was examined by use of three freshwater organisms(D. magna, O. latipes, and P. subcapitata). The effluent from HWE shows no toxicity. However, the effluent from HTT gave an obvious toxicity, and the degree of toxicities was increased as the elevation of HTT temperature. The toxicity of 350℃-HTT effluent was comparable to those for phenols when the three freshwater organisms were tested. Also, the degree of toxicity was discussed in terms of the molecular weight and chemical structures of organic component present in the effluent.