Provided is a method for estimating the number of tags in a slotted Aloha-based RFID system, which can estimate the number of tags through a new statistical average scheme using the number of slots, the measured number of empty slots, and the measured number of ID slots. The estimating method includes the steps of: a) setting the number (N) of slots, the measured number (c0) of empty slots, and the measured number (c1) of ID slots as parameters; and estimating the number (n) of the tags by substituting the set values into n=(N−1)/(c0/c1).