The present invention relates to an automatic gain controller in a wireless packet communication system and an automatic gain control method thereof. The automatic gain controller includes a variable gain amplifier, a signal saturation determining unit, a power calculator, and a gain controller. The gain controller includes a first gain selector and a second gain selector. In the automatic gain control method, an establishment gain of a variable gain amplifier is reduced when the number of samples greater than a predetermined reference is greater than a predetermined number. After a received power value is measured, the establishment gain is controlled by a difference between the received power value and a reference power value.