The effects of hydrogen and prestrain on the tensile properties of a low carbon steel plate JIS-SGP (0.078 C-0.012 Si-0.35 Mn) were investigated. The steel gas pipeline material JIS-SGP, which is one of candidates for hydrogen gas pipelines, is produced from the low carbon steel plate by plastic working and welding. In order to charge hydrogen, the virgin, and 10 and 20% prestrained specimens for tensile tests and hydrogen content measurements, machined from the low carbon steel plates, were immersed in a 20 mass% aqueous solution of ammonium thiocyanate at 313 K for 48 h. The hydrogen content increased by hydrogen charge after prestraining. Thermal desorption spectroscopy showed that the diffusible hydrogen contents are 0.38 ppm for the virgin hydrogen charged specimen, and 0.43 and 0.71 ppm for the 10 and 20% prestrained hydrogen charged specimens, respectively. The reduction of area was not affected by prestraining in the uncharged specimens, but it was affected by prestraining in the hydrogen charged specimens. The reduction of area was lower in the hydrogen charged specimens than in the uncharged specimens. The residual diffusible hydrogen contents after tensile test in the hydrogen charged specimens were between 0.38 and 0.71 ppm. The reduction of area decreased linearly with increasing hydrogen content in the hydrogen charged specimens. Scanning electron microscopy showed that cup-and-cone fracture occurred in both the uncharged and hydrogen charged specimens, and that the fracture surfaces were covered by dimples. Dimples were small in diameter and deep in the uncharged specimens, while they were large and shallow in the hydrogen charged specimens. Many voids were observed on the longitudinal section of tensile fracture specimen. The voids were elongated along the tensile axis in the uncharged specimens. On the other hand, the voids were elongated in the direction perpendicular to the tensile axis in the hydrogen charged specimens. It can be concluded that hydrogen enhanced slip deformation ahead of the void edge at the equatorial plane and results in a flat ellipsoidal void in the direction perpendicular to the tensile axis. This unique mechanism of development of dimples and voids in the hydrogen charged specimens should be noted to understand the basic mechanism of hydrogen embrittlement.