An apparatus for adaptively decoding based on a channel variation at a receiver of a communication system supporting an Alamouti mode is provided. The apparatus includes a channel variation estimator for estimating the channel variation; a channel estimator for estimating a channel value of successive channel periods or a channel value of each channel period by comparing the estimated channel variation with a threshold; an Alamouti decoder for decoding a receive signal using an Alamouti method when the estimated channel variation is less than or equal to the threshold; and a Spatial Multiplexing (SM) decoder for decoding the receive signal using an SM method having a square channel matrix when the estimated channel variation is greater than the threshold.