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Recently, production of high frequency electric resistance welding (HFW) pipes has progressed rapidly, and application of HFW pipes to line pipe is becoming increasingly common. In addition to the welding technology itself, quality assurance of the weld seam of the HFW pipe is very important for application to line pipe as a substitute for UOE pipes or seamless pipes.1–4) The mechanical test and flattening test are applied to quality assurance of mechanical properties. The Charpy impact test is used to evaluate low temperature toughness, but that test is a sampling inspection and requires destructive testing. It has been pointed out that there is a correlation between the area of a penetrator in the ductile fracture area in the cross section of the Charpy impact test piece and the transition temperature.5–7) A penetrator is an oxide generated during HFW. Ordinary penetrators are elliptical in shape and about 0.2 mm to 5 mm in size. It is possible to detect penetrators of mm order size by the conventional ultrasonic test. However, as a result of progress in HFW steel pipe manufacturing techniques, penetrators of mm order size are now rare. The present problem in manufacturing HFW steel pipe is control of very micro scale penetrators. In order to investigate the form of micro scale penetrators that influence the results of the Charpy test, analysis and Evaluation Method for Low Temperature Toughness of Weld Seam of HFW Pipe Based on the Distribution of Scattered Type Penetrator