Using a new apparatus which can restrict the expansion and contraction of a synthetic specimen at an arbitrary temperature during thermal cycles, the hot cracking tendency of weld HAZ was examined on three high strength steels. The main results are summarized as follows: (1) In a test steel Z(HY-130 class), hot cracks occurred in the condition that a synthetic specimen was heated to above 1,350•Ž and was restrained within the temperature range of 1,350 to 1,380•Ž.