GaN and its related nitride materials have been investigated for many years due to their extensive applications in semiconductor optoelectronics and microelectronics. The realization of p-GaN plays a key role in developing the GaN-based optoelectronic devices such as light-emittingdiodes, laser diodes and ultraviolet photodetectors. Furthermore, it is very significant to acuurately obtain the carrier concentration value of p-GaN layer for device design and fabrication. Usually the Hall measurements are employed to obtain the hole concentration of p-GaN layer. However, this method is not suitable for very thin samples, especially the p-GaN layer in the device structure, which is commonly very thin. Furthermore, the good Ohmic contact to p-GaN is not easy to realize. In consideration of the importance of p-GaN in determining the performance of GaN-based devices, it is necessary to find other new methods to measure or check the carrier concentration data of p-GaN. In this paper, a new method to estimate the carrier concentration of p-GaN by analyzing the current-voltage characteristic curve of p-GaN/n+-GaN diode is proposed. The main physical process is as follows: generally the carrrier concentration of p-GaN layer is far less than that of n+-GaN layer, and the depleted region is mainly located in the p-GaN. When the reversed bias voltage is very small, the diode shows conventional properties of p-n+ junction and the corresponding reversed current is very low since the p-GaN is not completely depleted. With the increase of reversed bias voltage, the depleted region of p-GaN also increases. Once the p-GaN is completely depleted, the case turns different. The diode will show Schottky junction properties and the corresponding reversed current increases obviously when the p-GaN is completely depleted under a certain reversed bias voltage since the ideal reversed current of Schottky junction is larger than that of p-n+ junction. The hole concentration could be derived according to the device physics if the bias voltage is discovered, which leads to the properties changing from the p-n+ junction to conventional Schottky junction. The simulation results confirm the idea, and the calculated p-GaN carrier concentration is almost equal to the originally assumed value. The proposed method is interesting and may be helpful to accelerate the research of p-GaN and related optoelectronic devices.