Low-yield point steel has been used in seismic control dampers due to its high energy absorption ability. Meanwhile, thermal cycle during welding can change mechanical properties and microstructure of low-yield point steel. In addition, large plastic strains caused by seismic motion can induce defects in microstructure. They can cause the reduction in toughness of the low-yield point steel. In this study, Charpy impact tests were performed for low-yield point steel with thermal cycle simulating heat affected zone and preliminary plastic strain. Significant reduction in Charpy absorbed energy was observed when applying simulated thermal cycle, while the preliminary strain gives insignificant reduction.