The composite coating was prepared by thermal spray welding after making composite powder,which is composed of Ni-based self-melted alloy and Al_2O_3 ceramic powder including nano,sub-micron and micron powders.The influences of contents and sizes of Al_2O_3 on the structure and wearability were investigated.The results show that the wear resistance of the coating would be increased greatly by adding Al_2O_3,but the spray weldability decreases with increasing Al_2O_3 content.So there is an optimal content of Al_2O_3 powder.The composite coating with Al_2O_3 nano or sub-micron powder of 0.5% has the best abrasive resistance,while the optimal content of Al_2O_3 micron powder is 1 %. The composite coating was prepared by thermal spray welding after making composite powder, which is composed of Ni-based self-melted alloy and Al 2 O 3 ceramic powder including nano, sub-micron and micron powders. These influences of contents and sizes of Al 2 O 3 on the structure and wearability were investigated. The results show that the wear resistance of the coating would be greatly increased by adding Al 2 O 3, but the spray weldability decreases with increasing Al 2 O 3 content. There is an optimal content of Al 2 O 3 powder. The composite coating with Al 2 O 3 nano or sub-micron powder of 0.5% has the best abrasive resistance, while the optimal content of Al 2 O 3 micron powder is 1%.