In this paper, deep level defects in undoped and Si–doped GaN have been studied using digital deep level transient spectroscopy. Common trap levels at Ec -ET ∼ 0.15-0.20 eV and 0.59-0.62 eV were detected for both undoped and Si-doped samples. For the doped samples, three additional defect levels at Ec -Et ∼ 0.11, 0.28, and 0.45 eV were detected. The concentration of the 0.15-0.20 eV was found to be much higher in undoped GaN that also shows higher dislocation density. Based on this correlation and the logarithmic capture behavior observed for this level, indicative of extended defect nature, we attribute the 0.15-0.20 eV level to dislocation related defects. On the other hand, the 0.28 and 0.45eV trap levels are tentatively attributed to Sirelated defects simply due to the fact that these two levels were observed only in Si-doped GaN. The 0.11eV trap level, which exhibits an exponential capture kinetic, is believed to be related to nitrogen vacancies.