A 5 KW CO₂ laser has been used to modify the surface microstructure of Al–Mg–Si aluminum alloy with pure Ni and Ni–Cr–B–Si powders. The experimental results indicate that a porosity-free zone can be generated after laser surface alloying (LSA). The Al₃Ni particles were only observed in the LSA Ni sample. In the LSA Ni–Cr–B–Si sample, there are three regions indicated as surface region in the melt zone (region A), bottom region in the melt zone (region B), and amorphous phase in bottom region (region C) from the top to the bottom. In the region A, the Al₃Ni particles are present and the Al₃Ni₂ particles distributed in the region B. The partial Al–Ni–Cr amorphous structures are dispersed in the region C. And, the AlNi and Al₈Cr₅ structures were observed in the interface of region B and C. The hardness of the LSA Ni–Cr–B–Si sample is sharply increased than Al-matrix and Ni specimen, specifically the amorphous structure in region C is about 18 times higher than the Al-matrix.