Disclosed is a power control method in a portable Internet system for controlling data transmission output on a mobile communication system by using an AMC table in a mobile communication network employing an AMC scheme, the AMC table including modulation levels of data transmitted on the mobile communication system and request power information according to the modulation levels. The power control method includes the steps of: (a) setting maximum request power of request power corresponding to the modulation levels allocated to the mobile communication system as data transmission output on the mobile communication system by means of the AMC scheme; (b) comparing a BER occurring in the data transmission with a preset critical BER; (c) decreasing the data transmission output by a predetermined interval when the BER is smaller than the preset critical BER; (d) increasing the data transmission output by a predetermined interval when the BER is larger than the preset critical BER; and (e) maintaining the data transmission output at a current state when the BER is equal to the preset critical BER.