Various notch toughness and weldability tests were performed for Si-Mn Y N D steel which was developed as the economical material for low temperature applications. The material was Si-Mn Aluminum-killed steel in the pressure quenched and tempered condition.The testing results summarized :1. According to the results of various Charpy impact tests, the notch toughness of the steel is much superior to that of ordinary Aluminum-killed mild steel.2. The notch toughness of the material is less affected by cold-work, strain aging and reheating at elevated temperatures that of ordinary Aluminum-killed mild steel.3. According to the tensile double testing results, the steel plate of 12mm thickness has shown that the upper critical temperature for brittle fracture propagation is -52°C under the applied stress of 15 kg/mm2.4. The welded joint of the steel shows satisfactory tensile strength, ductility and fatigue strength when it is welded with 2.5% Ni electrode.5. The I. I. W. maximum hardness test, Kommerel test, slit type restricted cracking test and C. T. S. test of the material have shown that the material has as satisfactory weldability as that of ordinary Aluminum-killed mild steel.6. The notch tensile test at low temperatures was performed to study the characters of the initiation of brittle crack at welded bond, using wide plate test pieces with cruciformly welded joints. The upper limit of temperature for brittle fracture initiation was -110°C, which is low enough for practical use.