Abstract: Natural gas leakage accident occurred in a high-frequency electric resistance longitudinal welded pipe. The leakage cause of the welded pipe was analyzed by macro-examination, non-destructive testing, chemical composition analysis, mechanical property test, metallographic analysis, scanning electron microscopy and energy spectrum analysis. The results show that incomplete fusion defects were produced in the weld of the welded pipe because of the improper welding process, and were not found by missing inspection during the manufacturing of the welded pipe. Therefore the incomplete fusion defects developed and formed leak holes in service, finally leading to the leakage.