On weld cracking, especially on the cold cracking during cooling, intensity of restraint of weld joint is one of the important factors. Research was attempted to investigate the behavior of weld cracking under some restraining conditions and to obtain informations on the effect of reaction stress on weld cracking. A new test apparatus as shown in Fig. 1 was used for direct measurements of reaction stresses during cooling. Both forced and restraining tension tests were made by the apparatus. The former is such that a specimen is fractured by tensile load at a high temperature during cooling, and the latter is such that the length of a specimen is kept constant during cooling. Experiments were carried out on both weldments of a mild steel and a high strength steel of 60 kg/mm² tensile strength level. High strength steel weldments have higher fracture stress and elongation than those of mild steel weldments in forced tension tests during cooling. In restraining tension test, however, reaction load increases gradually with cooling, and higher intensity of restraint usually favours crack initiation and decreases the time required for cracking. At first, micro crack occurs at root of weld, after which it intermittently develops with cooling. In higher intensity of restraint, it develops to complete fracture before the weldment is cooled to room temperature. In most of the specimens, however, the development of the micro crack stops without complete fracture and constant reaction load is kept at room temperature. After such specimens were kept at the constant reaction loads for 2 or 3 days, no more development of the micro cracks was observed. Micro crack initiation occurs at a lower reaction stress as compared with fracture stress obtained by forced tension test, and the larger difference between them is observed for high strength steel weldments.