Self-assembled InAs nanocrystals were grown by droplet epitaxy on a GaAs(1 0 0) substrate at a relatively low growth temperature of 160 degrees C. The growth mode of InAs varies as a function of the surface stabilizing conditions of the GaAs(1 0 0) substrate. Three-dimensional InAs nanocrystals with a height of 30-40 nm and a base size of 350 nm were grown on the Ga-stabilized surface. However, a two-dimensional layer of InAs with a root-mean-square roughness of 1.4-3.5 Angstrom was formed on the As-stabilized surface. The island density was higher on the vicinal surface than that on the singular surface. The islands were preferentially aligned along the < 1 0 0 > direction, i.e., parallel to the step edges of the substrate. The base edges of islands on the vicinal surface were also aligned along the < 1 0 0 > direction. These results suggest that the conditions of substrate surface, as well as the strain energy due to the lattice misfit between InAs and GaAs play important roles in the formation of self-assembled InAs nanocrystals by droplet epitaxy.