The purpose of this study was to improve the adsorption ability of activated carbon for removal of trihalomethanes (THMs) to design a small and efficient water purifier. The phenol activated carbon with the high adsorption ability to THMs was used in the experiment. To improve the adsorption ability of the activated carbon, the effect of heat treatment temperature under the nitrogen atmosphere was investigated. The effect of the heat treatment temperature was evaluated from the chloroform adsorption isotherm, and the amount of purified water containing THMs. First of all, phenol resin was carbonized at 600 degrees C for 3 h, and was activated by steaming at 900 degrees C. Activation time was 0.5, 1.0, and 2.5 h. Next, the heat treatment was carried out for the sample of 1.0 h activation with the highest adsorption ability. The heat treatment temperature was 450, 600, 750, and 900 degrees C. The heat treatment time was fixed at 3 h for all samples. As a result, it was found that the adsorption ability of the phenol activated carbon to THMs raised by about 1.6 times with the heat treatment at 750 degrees C following the activation. On the other hand, the influence of the heat treatment on the structure of the phenol activated carbon was investigated. The structure was analyzed with pore size distribution measurement, X-ray photoelectron spectroscopy (XPS), temperature programmed desorption mass spectrometry (TPD-MS), and field emission-type scanning electron microscope (FE-SEM). The elimination of functional groups were observed clearly with the heat treatment, which means increased hydrophobicity of pore surface in the phenol activated carbon. Moreover, enlargement of the pore entrance were confirmed. Therefore, we concluded that THMs were efficiently led into pores due to the enlarged pore entrance, and was adsorbed strongly due to the increased hydrophobicity.