Radio Frequency Identification (RFID) is a method for automated identifying objects. One of the problems of this technology is its security. RFID tags include resource limitation; therefore, the system designers cannot implement complex circuits to enhance their security. Usually the symmetric and asymmetric encryption methods increase resources and cost. Because it is believed to increasing security is inconsistent with the simplicity, the researchers mostly use one-way encryption methods. In this paper, we propose a mutual authentication protocol based on public key cryptography. The used encryption method includes high security and low complexity. This protocol performs in few steps and is suitable for portable devices with power limitation. In terms of security, the proposed protocol is robust against known attacks. In addition, we prove the protocol is secure by an analytical method.