Tag collection is one of the major functions in radio frequency identification (RFID) systems. ISO/IEC 18000-7 defines the tag collection algorithm using the anticollision algorithm, based on the framed slotted ALOHA for active RFID systems, but it has efficiency problems that reduce tag collection performance. This paper focuses on improvement of tag collection performance in active RFID systems that comply with ISO/IEC 18000-7. To overcome the efficiency problems and improve tag collection performance we propose two mechanisms: (1) a new slot size decision mechanism to allow the reader to choose the optimum slot size flexibly and (2) a broadcast-based sleep mechanism to put collected tags to sleep effectively. These mechanisms require modification of the standard tag collection algorithm so that they can be readily applied to standard-compliant active RFID systems. We also implemented an active RFID system, composed of an active RFID reader and multiple tags, supporting the modified tag collection algorithm as well as the standard tag collection algorithm. The reader is designed to maximize tag collection performance when the proposed mechanisms are applied. Experimentally, we evaluated the tag collection performance using the reader and 50 tags in the real-world environment. The experimental results show that when two mechanisms are applied and the initial number of slots is chosen appropriately, the performance of the modified tag collection algorithm is greatly enhanced compared with the standard tag collection algorithm: it requires only 945 ms for the reader to collect all 50 tags on average, which is 534 ms less than the 1479 ms taken by the standard tag collection algorithm.