Specular single-crystal InP epilayers have been grown directly on Si(100) substrates by low-pressure organometallic vapor phase epitaxy. The effects of the initial nucleation process on the structure properties of the films were investigated, and improvements in the growth technique leading to higher quality InP films are reported. The InP/Si epilayer grown under optimum conditions exhibits high optical quality compared with that of the InP homoepilayer. Post-growth thermal annealing at 780°C was also confirmed to be effective in improving the overall quality of InP-on-Si. The results presented are superior to those reported previously for InP/Si heteroepitaxy.