To obtain suitable process conditions for diffusion bonding of two types of steel, Hot-pressure bonding and Hot-roll bonding techniques are studied. High carbon steel and low carbon steel are bonded by Hot-pressure bonding and Hot-roll bonding with Auto-vac. cleaning process. GTAW welding method was employed to seal the bond surface in Hot-pressure bonding and SMAW welding was employed in Hot-roll bonding. The bond strength was increased by the alloying effect of the diffusion of carbon. In Hot-pressure bonding, the bond strength was increased with the bonding temperature and it was related to bonding time by the following equation. ln(bond strength)＝A·t^(½)＋B In Hot-roll bonding, the bond strength was related to rolling reduction ratio(R) by the following equation. bond strength＝C·R(2-R) The bonding process variables to obtain suitable bond strength are summarized and the possible bond mechanisms are discussed.