Initial growth of GaP on Si substrates using metalorganic vapor phase epitaxy was studied. Si substrates were exposed to PH₃ preflow for 15s or 120s at 830°C after they were preheated at 925°C. Atomic force microscopy (AFM) revealed that the Si surface after preflow for 120s was much rougher than that after preflow for 15s. After 1.5nm GaP deposition on the Si substrates at 830°C, GaP islands nucleated more uniformly on the Si substrate after preflow for 15s than on the substrate after preflow for 120s. After 3nm GaP deposition, layer structures were observed on a fraction of Si surface after preflow for 15s. Island-like structures remained on the Si surface after preflow for 120s. After 6nm GaP deposition, the continuity of GaP layers improved on both substrates. However, AFM shows pits that penetrated a Si substrate with preflow for 120s. Transmission electron microscopy of a GaP layer on the Si substrate after preflow for 120s revealed that V-shaped pits penetrated the Si substrate. The preflow for a long time roughened the Si surface, which facilitated the pit formation during GaP growth in addition to degrading the surface morphology of GaP at the initial growth stage. Even after 50nm GaP deposition, pits with a density on the order of 10⁷cm^-2 remained in the sample. A 50-nm-thick flat GaP surface without pits was achieved for the sample with PH₃ preflow for 15s. The PH₃ short preflow is necessary to produce a flat GaP surface on a Si substrate.