Radio frequency identification (RFID) technology is becoming increasingly attractive because of its high storage capacity and reprogrammability. There is a challenge to be overcome when a reader needs to read a number of tags within the reader's interrogation zone at the same time. In this paper, we present an anti-collision scheme in a RFID system. The scheme is based on the dynamic framed ALOHA protocol developed for radio networks. In our scheme, we propose two methods to estimate the number of tags. Simulation results indicate that the total number of time slots for reading all tags is about 4 times the number of tags that need to be read, including acknowledgement time slots. The main advantages of our scheme are the great performance of uplink throughput and its easy implementation for both readers and tags.