1. A method of recognizing a plurality of RFID tags in an RFID reader, comprising: obtaining an expected frame size in the case where throughput obtained by reflecting a slot length based on the number of remaining 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 predetermined frame size, wherein the throughput obtained by reflecting the slot length is obtained by a rate of a success slot with respect to all the success, collision, and idle slots using the success, collision, and idle slot lengths, and wherein the throughput is defined by the following equation, TH ⁡ ( n , p ) = T S ⁢ P S T S ⁢ P S + T C ⁢ P C + T E ⁢ P E in which TH is the throughput which is a function of n number of tags and p access probability, P S is a probability that the success slot will be generated, P E is a probability that the idle slot will be generated, P C is a probability that the collision slot will be generated, Ts is a success slot length, Tc is a collision slot length, and T E is an idle slot length.