The effective surface treatment method for steel insert composited with Al base metal by expendable pattern casting (EPC) process and the bonding interface between steel insert and Al base metal were investigated.It was found that Zn plating on steel insert was effective on improving the bonding property between steel insert and Al base metal in EPC process.Zn is thought to promote the formation of diffusion layer.But almost none content of Zn was observed in the boundary which had been plated on the steel insert.A diffusion layer consisting of Al,Si and Fe was formed at the insert/alloy interface and its hardness was higher than the steel insert as matter of course Al base metal.This layer turned out to be intermetallic compounds of Al-Si-Fe system.Higher pouring temperature promoted the diffusion of Fe into Al alloy,so Fe content in intermetallic layers increased at higher pouring temperature.The layer nearest to steel disappeared due to applied pressure. The effective surface treatment method for steel insert composited with Al base metal by expendable pattern casting (EPC) process and the bonding interface between steel insert and Al base metal were investigated .It was found that Zn plating on steel insert was effective on improving the bonding property between steel insert and Al base metal in EPC process. Zn is thought to promote the formation of diffusion layer. Bit almost none content of Zn was observed in the boundary which had been plated on the steel insert. A diffusion layer consisting of Al, Si and Fe was formed at the insert / alloy interface and its hardness was higher than the steel insert as matter of course Al base metal. This layer turned out to be intermetallic compounds of Al-Si-Fe system .Higher pouring temperature promoted the diffusion of Fe into Al alloy, so Fe content in intermetallic layers increased at higher pouring temperature. The layer nearest to steel disappeared due to applied pressure.