The heat affected zones (HAZ) of three kinds of high tensile steels were reproduced with a synthetic thermal cycle apparatus, and their notch toughness was evaluated by fracture stress curve test and Double Blow test in respect to the initiation of brittle fracture. As the results of the experiments it was found that the synthetic HAZs were in general remarkably notch sensitive, and the results of both tests had the same relation as previoulsly reported on many other base plates. The transition temperatures of synthetic HAZ were compared with the ones of bead welded specimen in the first report. It was revealed that Tr_α5 in Kinzel test and ( Tr_Ei ) ₂ in the synthetic HAZ varied almost in similar way with the increace in the hardness. On the other hand, a close corelation was not found between ( Tr_Ei ) ₂ in the synthetic HAZ and Tr₁₅ in the bead welded specimen. Finally, the behavior of brittle fracturing of bead welded specimen was schematically discussed, and it was concluded that the existence of thin and brittle layer as seen in HAZ would obviouly have adverse influence on the notch toughness of the specimen, for the range of the temperature lower than Tr_S of the base plate.