Abstract: Since the dilution rate in laser cladding of thin Ni alloy layers is too high,in this study thin Ni alloy cladding layers were prepared on 304 stainless steel using two Ni Cr Fe BSi powders with different particle sizes at low heat input and high scanning speed,and the effects of particle size and cladding parameters on cladding metallurgy reaction,microstructure and composition of the cladding layer were investigated.The results reveal that the thin powder bed can be fully melted and heated to a temperature high enough to complete the self-fluxing reaction with the low heat input,the alloy spreads adequately on the substrate,forming a flat cladding layer with little cracking. Using a Ni alloy powder bed of about 200 μm thick consisted of particles with the size of 70 ～ 100 μm,the thickness of the obtained cladding layer is about 170 μm and the dilution rate is relative high. Using the Ni alloy powder bed of the same thickness consisted of particles with the size of 50 μm,a cladding layer with a thickness of 130 μm was obtained,the dilution rate is lower and the cladding layer/substrate interface is more flat. Since the absorption rate of finer powder to laser energy is higher,it can be melted at lower heat input,the heat transfer to the substrate is restrained,thus melting and heat effect of the substrate is less serious. This method can be used in preparation of thin protective layer and precise repairing.