The use of an abundant explosive energy is one of the successful method to investigate the behavior of the brittle fracture of welded structures. Authors have previously conducted the explosive tests on arc welded mild steel tubes. Recently, however, another phase of study have been made carring out series of explosion tests on the arc welded tube specimens made of high tensile strength steel (“50 kg/mm² Mn-Si steel”, “60 kg/mm² V-Ti alloy steel” and “quenched and tempered steel”). The purpose of this investigation is the examination of such various effects on the behavior of brittle fracture as that of the quality of steel, welding procedure (manual and submerged welding), stress relieving after welding (600°C, 1 hour heat treatment, the Linde method and peening). Codes and numbers of specimens are listed in table 1, and the chemical and mechanical properties in table 2. The size of specimens were 400 mm diameter, 800 mm height and 20 mm thickness. The explosive (T. N. T.) was set in the center of a specimen filled up with water and detonated. The relation between test temperature and circumferential strain at fracture for different series of steel codes are shown in figs. 5 to 7, respectively. Detailed sketches of the fractured specimens are also shown in fig. 8. Figs. 9 to 11 show the relation between test temperature and averaged thickness of shear lips appeared on the fractured surface in the three kinds of base metal, respectively. Similar relationship was observed in weld metal and heat affected zone as shown in fig. 12. It can be seen from fig. 13 that the former is larger than the latter concerning to the thickness of shear lips. Summarizing the test results the following conclusions are obtained. 1. Comparing the as-welded tested specimens with respect to the effect of the quality of steel on the circumferential strain at fracture, it can be concluded that 50 kg/mm² high tensile steel is the best, and then comes quenched and tempered steel, 60 kg/mm² high tensile steel. And concerning the thickness of shear lips, quenched and tempered steel comes first and then 50 kg/mm² high tensile steel, 60 kg/mm² high tensile steel. In other words 50 kg/mm² high tensile steel is superior for the initiation of fracture, and the quenched and tempered steel for the propagation of fracture. 2. So far as the circumferential strain at fracture the effect of stress relieving heat treatment has not been remarkably observed. But after heat treatment the possibility of crack initiation at circumferential weld can be considered to increase. It may be the effect of the increased brittleness due to heat treatment on weld metal deposited by low hydrogen type electrodes. 3. The effect of the low temperature stress relieving by the Lide method was not appreciable. 4. In all the tested specimens peened on every layer of the outer surface, the crack initiated on the outer surface of the circumferential weld, and the strain at fracture was not so large. The peening on the last layer can be considered to produce extremely fine cracks and strain hardning of weld metal consequently crack will start more frequently at low strain level in spite of rich ductility the base metal. From the viewpoint of the brittle fracture peening on the last layer should be positively avoided. 5. The difference due to manual and submerged arc welding was no appreciable. 6. With the exception of peened specimens cracks were usually initiated from the inner surface of the pipe, especially from the heat affected zone of longitudinal seam joints, or blow holes in circumferential weld.