A radio resource allocating method in an adaptive antenna system is provided. The radio resource allocating method includes selecting one user when there are one or more users allocable to a current spatial frame; calculating a sum of Modulation order Product code Rate (MPR) values with respect to users allocated a same radio resource including the selected user when a total transmit power of a Base Station (BS) is divided by a current spatial frame index; and allocating the selected user to the current spatial frame by taking into account the calculated sum and a sum of MPR values with respect to users allocated to a previous spatial frame. Accordingly, a system throughput or a scheduler performance can be maximized.