In a mobile communication system, a method of setting a reverse activity bit (RAB) is provided. The method comprises measuring a rise over thermal noise-measured (ROTm) representing a load degree of a reverse link; comparing the ROTm with a setup reference value (ROTm_th); setting the RAB to lower data rate of a terminal, when the ROTm is greater than the ROTm_th; enabling a base station to receive and monitor a variation state of the ROTm, when the ROTm is less than the ROTm_th; and setting the RAB to control the data rate according to the variation state of the ROTm.