This paper proposes a novel tag collection algorithm for improving the performance of iterative tag collections in active RFID systems. In the proposed tag collection algorithm, a reader assigns fixed response slot numbers to the tags newly identified via a tag numbering phase, and then collects additional data sequentially from all the tags without tag collisions in the following tag collection phase, in which the tag responds in its own assigned slot. This lowers the cost of empty slots and collided slots during iterative tag collections. The simulation results showed that the proposed tag collection algorithm could achieve higher tag collection performances as the tag mobility was lower, compared with the standard tag collection algorithm and the modified tag collection algorithm that was proposed in a previous study.