We demonstrate a novel synthetic scheme that can be used to control the crystalline phase and shape of GaP semiconductor nanocrystals. Our study shows that steric effects of surfactant ligands can modulate the crystalline phases and control the shapes of nanocrystals. The shape of the nanocrystals obtained varies from zero-dimensional spheres to one-dimensional rods via controlling the ratio between primary and tertiary alkylamines. III-V semiconductors (in our case: GaP) under 10 nm in width are first reported, and unique optical properties due to shape anisotropy are also observed.