Dynamic framed-slotted ALOHA (DFSA) is one of the most popular algorithms to resolve tag collision in RFID systems. In DFSA, it is widely known that the optimal performance is achieved when the frame size is equal to the number of tags. So, a reader dynamically adjusts the next frame size according to the current number of tags. Thus it is important to estimate the number of tags accurately. In this paper, we propose a novel tag estimation method for DFSA. We compare the performance of the proposed method with those of other existing methods. And, simulation results show that our scheme improves the accuracy of tag estimation and the speed of tag identification.