In these days, high-strength coil springs for construction machines are very much in demand. It is necessary for the coil spring steel to have high hardness and high toughness to meet this needs. Newly developed spring steel has higher Charpy impact value than SUP11A due to the addition of V and the reduction of C. Generally speaking, springs for construction machines are bigger than those for automobiles. This study supported the theory that when the parts size becomes bigger, Charpy impact value decreases since cooling rate becomes slow in the quenching operation. However, newly developed steel has higher Charpy impact value than SUP11A even if the cooling rate is slow.