High-quality GaN films are prepared by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). In situ optical reflectance traces reveal that the ramping time from low temperature to high temperature in the growth of the nucleation layer significantly affects the surface roughness. Atomic force microscopy (AFM) images demonstrate that the increased ramping time improves the surface roughness and the quality of GaN films. Moreover, the surface roughness also influences the lateral growth and coalescence step. Furthermore, the dependence of the Hall measurement on temperature is investigated. The mobility of the lightly doped GaN is as high as 622 cm2/V-s at a corresponding carrier concentration of 1.1 × 1017 cm-3. The results of this study demonstrate that the proposed GaN films are of high quality and have the excellent electronic property.