In this study, diffused carbon in workpiece surface with Plasma Electrolysis Carburizing (PEC) treatment applied to harden SAE 1015 steel is investigated. The workpiece is connected cathodically to a high current DC power supply that applies high voltage and uses stainless steel as the anode. After water ionization with this voltage, a hydrogen film forms around the workpiece. This film acts as a thermal and electrical insulation layer which increases the electric resistance around the workpiece and makes it being heated. The surface of the workpiece is transformed to austenite and after the disconnection of the current, the workpiece is quenched in aqueous solution. By the aid of this method, the workpiece can be hardened to 1200HV or more. In this paper, the effect of time and solution concentration on the depth of diffused carbon was studied. The hardness of workpiece was measured and the variation of the diffusion depth versus time and solution concentration was investigated.