In this study, the relationship between the heat cycle and the Charpy absorbed energy is defined by simulating various heat cycles by performing the synthetic HAZ test using steel with a relatively high Charpy absorbed energy and steel with a distinctly low Charpy absorbed energy. On the basis of this relationship, the authors have studied the effects of the steel toughness on the Charpy absorbed energy of weld HAZs inputted the several type of welding heat cycle. As the results, the Charpy absorbed energy of the CGHAZ dropped in the case of single-pass welding, but the Charpy absorbed energy increased considerably when the maximum temperature decreased, irrespective of the steel toughness. The Charpy absorbed energy of the weld interface in multi-layer welding was greatly susceptible to the achieved temperature after the maximum achieved temperature, and the range of the temperature after the maximum temperature was achieved in which the Charpy absorbed energy was recovered was found to be different depending on the steel toughness.