In this paper, an improved small-signal equivalent model is introduced to eliminate frequency dispersion phenomenon in capacitance-voltage (C-V) measurement, and a new mathematic method is proposed to calculate the amount of bulk defect existing in the GaN buffer layer. Compared with photoluminescence (PL) and high resolution X-ray diffraction (HRXRD) data, it is identified that the main component of the bulk defect concentration is made up of the point defect concentration, rather than the edge dislocation concentration. All these results prove the accuracy of the improved C-V model and feasibility of the mathematic model. (C) 2011 Elsevier Ltd. All rights reserved.