Following phenomena were experienced on the welding with plate thickness 100 mm to 200 mm.(1) The weld cracking increases with an increases of the plate thickness (number of welded layer).(2) Weld cracking occurs several days after welding.In order to assure above phenomena and study the preventive method of the weld cracking, the weld cracking test was carried out and the weld cracking phenomena were observed.The main results obtained are as follows.(1) It is clarified that weld cracking increases with an in crease of the thickness of welded plate, and the initiating position of weld cracking lies just under the surface bead.(2) Incubation period of weld cracking is 4 days after welding.(3) Weld cracking is prevented with lower temperature postheating for 0.5 hr at 300°C in case of local gas heating for a 2 1/4 Cr-lMo steel.(4) It is clarified that above phenomena are based on the distribution of hydrogen in thick weldment which is reported by authors in the previous reportt.