A method of recognizing a tag in an RFID reader includes: obtaining a predetermined frame size in the case where throughput obtained by reflecting a slot length based on the remaining number of tags except for recognized tags within a frame progressing current tag recognition that substantially has a maximum value; and when the current frame size is different from the predetermined frame size, stopping the current progress of frame and changing the current frame size into the expected frame size. The method of recognizing a tag may further include when the current frame size is equal to the expected frame size, changing the current frame size into the frame size that does not cause a throughput inversion phenomenon for the remaining number of tags to be recognized within the frame progressing the current tag recognition. By the throughput inversion phenomenon, the occurrence of throughput loss can be prevented and the tag recognition time is reduced to recognize the tags at high speed.