Ni-Co-P/Si3N4 composite coating on Al alloy is prepared by electroless composite plating in order to improve the minohardness of Al alloy. The influence of electroless plating process parameters on the micro-hardness of Ni-Co-P/Si3N4 coating on a hyper-eutectic Al-Si casting alloy is studied by an orthogonal experiment. The effects of process parameters, such as pH and mole ratio of Co 2+/（Co 2+ +Ni2+）,Si3N4 concentration in electroless plating bath and plating temperature,on the micro-hardness of composite layer are obtained. It is indicated that the hardness of coatings can be increased by increasing pH, plating temperature,Si3N4 concentration and keeping mole ratio of Co2+/（Co 2+ +Ni2+）moderate. Moreover, the influence of heat treatment on the micro-hardness of coatings is also investigated. It is found that the hardness of films increases with the heat treatment temperature rising below 400 ℃ but decreases when temperature is over 400 ℃.