Recently, GaN has attracted much attention for short wavelength LEDs, LDs and future opto-electronic integrated devices. In our study, Boronmonophosphide(BP) grown on Si(100) by MOCVD is used for growing cubic-GaN (c-GaN). The epitaxial growth of GaN/BP/Si has been carried out. We observed the crystal defects by scanning electron microscope(SEM), transmission electron microscopy(TEM), and surface X-ray diffraction(XRD). The dislocation density in the BP layer markedly decreased with increasing the thickness. However the difference of the lattice constant between BP and Si leads to high dislocation densities in GaN layer. Therefore, the epitaxial growth of Si doped BP on Si has been carried out, in order to observe the effect of Si doping on BP crystalline quality. We observed the crystal defects by XRD and cross-sectional TEM. The dislocation of interfaces will be discussed.