Additive manufacturing (AM) has garnered substantial attention owing to its capability to fabricate parts with complex shapes. In addition, the powder-bed fusion (PBF) type AM process can also control internal microstructures by controlling the rapid cooling conditions unique to the PBF process [1]. On the other hand, we found by the phase field simulation that the rapid heating condition during the melting process also affects the microstructure formation in Al-Si eutectic alloy [2]. Si crystalline particles remain near the fusion line even after the rapid melting process, serving as heterogeneous nucleation sites in the consequent solidification process. The crystalline Si particles are suggested to remain owing to the melting point of solid Si phases substantially higher than that of α-Al by approximately 750 K, and the short diffusion time during the rapid melting process. Our study also suggests that the intrinsic heterogeneous nucleation mechanism in Al-Si alloys can be utilized for novel grain refinement [3]. However, the heterogeneous nucleation cannot be explained by classical nucleation theory. It is supposed that the results and the theory do not agree because the crystallization occurs from a non-equilibrium heterogeneous liquid containing remaining Si particles due to the rapid heating and cooling conditions of the PBF process. Therefore, we conducted molecular dynamics (MD) simulations of heterogeneous nucleation of Al from the Si solid/liquid interface to clarify the heterogeneous nucleation behavior during the PBF process. We found that Al liquid forms an ordered structure near the solid Si/liquid Al interface even above the melting temperature of Al, and the development of Al crystals started from the ordered region. The results suggest that the ordering of Al liquid near the solid-liquid interface is a key to the frequently observed heterogeneous nucleation in the PBF process of the Al-Si eutectic alloy.