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In an RFID system, a tag can be identified by a reader when it moves into the interrogation zone of the reader. Collisions occur when tags using backscatter technology respond their IDs to the reader at the same carrier frequency simultaneously. The performance of tag identification is degraded by collisions, so it is important to reduce them. Several anti-collision protocols have been proposed for reducing tag collisions. They can be categorized into two classes: ALOHA-based and tree-based protocols. In this paper, we propose a 4-ary query tree-based anti-collision protocol, namely parallel response query tree (PRQT) protocol, to improve the performance of tag identification. In PRQT protocol, the tags with the ID prefix matching the reader's request bit string or the complementary of the string are arranged to respond in two sub carriers in parallel. We also conduct simulation experiments for the proposed protocol and compare it with related ones to demonstrate its advantages.