In this paper, the processing parameters of growing GaN epilayer by hydride vapor phase epitaxy are optimized. The influences of the low-temperature (LT) nucleation layer growth time, V/Ⅲ precursor ratio and the growth temperature on GaN layer are investigated by the high-resolution X-ray diffraction (HRXRD) signature for the asymmetric and symmetric reflections. The investigation finds that the LT-nucleation layer not only supplies the nucleation centers having good crystal quality, but also promotes the lateral growth of the sequent high temperature (HT) growth. The optimal LT nucleation layer growth time, V/Ⅲ precursor ratio and the growth temperature can effectively enhance lateral growth to reduce the crystal defects and are favorable to converting the growth mechanism from three-dimension to two-dimension in HT growth. The structural and optoelectronic properties of the as-grown GaN layer with a thickness of 15 μat the optimal parameters are studied by scanning electron microcopy, atomic force microscopy (AFM), HRXRD, Raman spectra, and photoluminescence (PL) measurements. X-ray rocking curves show that the full widths at half maximum of (002) and (102) are 317 and 343 arcsec, respectively. The surface roughness (rms: root mean square) is 0.334 nm detected using AFM. These characteristics show that the sample has good lattice quality and smooth surface morphology. In PL spectrum, the near band edge emission is dominated by emission from excitons bound to neutral donors (D0X) near 3.478 eV with 11 meV blue-shift and the yellow band emission is very weak. The results indicate that the GaN layer has good crystal quality and excellent optoelectronic properties, but a little biaxial in-plane compressive strain also exists in it due to the lattice and thermal mismatch.