Abstract: The effective diffusion coefficient of hydrogen in the low carbon steel was determined by means of hydrogen diffusion theory fitting method and the time lag method. The diffusion coefficient and the concentration distribution of hydrogen in the low carbon steel were analyzed by the partial decay and build up transients fitting. The complete current density decay can be used to determine the reversibly trapped hydrogen which was released from the anodic side of the specimens.The results indicated that part of the reversible hydrogen in the low carbon steel could be characterized by the complete current density decay at the membrane exit side. The diffusion of hydrogen in the low carbon steel was retarded and the diffusion coefficient was reduced because of the hydrogen trapping in the low carbon steel and slow surface processes on the entry side. The real diffusion coefficient and the concentration distribution of hydrogen in the low carbon steel were obtained by the partial decay and build up transients fitting.