Abstract: Disclosed is a method of controlling the operation of a plurality of compressors each having at least one capacity controller capable of changing the discharge rate of the compressor in a stepped manner in such a manner as to provide a total discharge rate just meeting the load demand which varies momentarily. In this method, the control system is divided into a capacity control loop for responding to comparatively small load variance and a compressor control loop for responding to a comparatively large load variance. The capacity control loop is further divided into a sub-loop for capacity controllers of a compressor which has worked longest and which is to be stopped first and a sub-loop for the capacity controllers of other compressors. The unloading operation is commenced preferentially with the sub-loop for the capacity controllers of the compressor to be stopped first, while the on-loading operation is made first with the sub-loop for the capacity controllers of other compressors.