Hypereutectic Al- 18Si alloy is remelted and solidified rapidly with a 2 kW CO2 CW laser under different power densities and scanning velocities. The microstructures in the treated zone are analysed with SEM and an electron microprobe analyser. From the analysis of experimental results and the coupled zone computed for the f-n f system of Al-Si alloy with a nonequilibrium theory, it is shown that if the conditions of solidification determined by power density and scanning velocity are established in the central part of the coupled zone, a full eutectic structure with an eutectic spacing below 200 nm can be obtained using laser surface treatment for hypereutectic Al-Si alloy.