The deep level spectra in a series of undoped GaN films with different GaN buffer growth rates were studied by deep level transient spectroscopy (DLTS), transmission electron microscopy (TEM) and high-resolution X-ray diffraction (HRXRD) in order to investigate correlation between dislocations and deep level defects. Two distinct deep levels were observed, one with an activation energy E1 = 0.19-0.23 eV and the other with an activation energy E2 = 0.50 - 0.60 eV below the conduction band. The broadening of the (102) Bragg peak with decreasing buffer growth rates was related to increasing edge dislocations and also the DLTS peak intensity of the deep level E1. The logarithmic dependence of the DLTS peak intensity on the filling pulse time indicated that the deep level E1 is associated with dislocation core sites.