The blue luminescence of unintentionally doped GaN at room temperature was studied. The unintentionally doped GaN films were grown on (0001) oriented Al

2

O

3

substrate by MOCVD. TMGa and NH

3

were used for Ga and N sources

respectively. N

2

and H

2

were used as carrier gases. Several analytical techniques were employed to characterize the grown layers. The optical properties of GaN films were measured by photoluminescence (PL)

the electrical properties were characterized by the Van der Pauw Hall method at room temperature

and the crystalline quality were analysed by double crystal X-ray diffraction (DXRD). The research results indicated that the blue luminescence (about 2.9eV) at room temperature in unintentional doped GaN was obviously related with the compensation ratio. The intensity of blue luminescence was strong in high compensation ratio of GaN

while it was weak in light compensation ratio of GaN. It is considered that the blue luminescence was related with acceptor levels. Further study showed that the peak position of the blue luminescence shifted to lower energy by about 35meV with increasing excitation density

and the peak intensity was superlinear with the excitation density. The blue luminescence in undoped GaN was attributed to the transition from the free electron in conduction band to acceptor levels (eA luminescence).The results indicate that the blue luminescence will be restrained and the band edge emission will increase by using a large flow rate H

2

in the main carrier gas.