The present invention relates to a control of a compressor (8) at a rock drilling apparatus, said rock drilling apparatus including a power source for driving a compressor (8) at a rock drilling process, said compressor (8) being arranged to operate according to a first mode and according to a second mode, wherein, in said first mode, the work produced by the compressor (8) is arranged to be controlled by controlling the rotation speed of said compressor (8), and wherein, in said second mode, the work produced by the compressor (8) is arranged to be controlled by controlling the air flow at the compressor inlet. The method includes determining a parameter value representing a demand of work from said compressor (8), controlling the compressor (8) according to said first mode when said parameter value representing a demand of work from said compressor (8) exceeds a first demand, and controlling the compressor (8) according to said second mode when said parameter value representing a demand of work from said compressor is lower man said first demand. The invention also relates to a system and a rock drilling apparatus.