Hydrogen attack of low carbon steels with 0.003-0.15%C heated at 380°C in 200 atm hydrogen was investigated with reference to the change of distribution of Fe₃C carbide in the steel. The susceptibility of the steels with carbides precipitated continuously at grain boundary to hydrogen attack was larger than the steels with carbides precipitated dispersively in the matrix. Many voids and micro-cracks were observed at the hydrogen-attacked intergranular fracture surface with a scanning electron microscope. The gas evolvedfromthe hydrogen-attackedfracture surface consisted of H₂ and CH₄ by gas chromatography. Non-diffusible molecular hydrogen and methane which couldn't be removed with the dehydrogenation at 250°C for 3 hr in vacuum, increased with increase of carbon con tent and internal defects. It is suggested from these results that hydrogen attack is caused with the voids which are produced with molecular hydrogen or methane trapped at grain boundary carbide.