Unintentionally doped GaN thick film grown on (0001) oriented Al2O3 substrate by hydride vapor phase epitaxy (HVPE) is studied in experiments, by absorption spectra and photoluminescence (PL) spectrum. The blue and yellow luminescence at room temperature are observed. The results indicate that when the frequency is low, the PL spectrum is contained by only band-side emission; however, when the frequency becomes high, there are three emission bands (band-side emission, blue emission and yellow emission). Meanwhile, the higher of the frequency, the larger of I1(intensity of band-side emission) /I2 (intensity of blue emission) and I1/I3 (intensity of yellow emission). After calculation and analysis, it can be considered that the blue luminescence is related with the impurity C of the sample, and the yellow luminescence is related with the structure defects like as dislocations.