This paper proposes an authentication protocol based on hash and AES safe from various types of attacks in order to assure the security of communication between tags and readers, which exchange data with each other wirelessly in a RFID system. The proposed authentication protocol resolves a problem in existing hash-based protocols whereby the same hidden value is generated for the same identification in each session. In order to hide tag identification information a number of complicated calculations were required, but using the proposed AES protocol reduces such calculations, strengthens security against replay attack, spoofing attack, traffic analysis, eavesdropping, etc. and assure mutual authentication between tags and readers.