In order to improve both the hardness and the erosion resistance of cast aluminum alloys, nickel-diamond compound coatings were deposited on the alloys by a compound electroless plating process. The morphology, phase structure, hardness, erosion resistance and adhesion of the electroless coating were investigated. The results show that the pretreatment such as removing the silicon in the surface of the alloys, zinc dipping, alkali electroless nickel is necessary. The deposition of an electroless nickel coating without diamond prior to nickel-diamond electroless plating can improves the erosion resistance. A best nickel-diamond compound electroless plating process is found. The hardness of the nickel-diamond compound coating reaches 730 HV. Both the adhesion and erosion resistance of the compound coatings are very good.