This paper studies the energy characteristics with respect to process parameters from a systematic point of view, in terms of electricity and materials. A detail analysis of energy characteristics of a CNC machining system is firstly presented, based on which the calculation models of energy efficiency are formulated. Then the effects of process parameters on energy and processing time are investigated by using S/N analysis. The results show different optimization trends for two kinds of specific energy consumption considered in this work and detail explanations of the trends are given afterwards.