This paper presents one tuning method of servo parameters of single axis servo drive system in NC machine tools. In this method, using computer simulation on mathematical model of feed drive system, servo parameters are tuned as maximizing servo stiffness. As a result, high servo stiffness was achieved than that can be given using standard servo parameters. In experiment, servo stiffness of existing feed drive system of machining center was measured and closely reflected the results of the computer simulation. It was also shown that droops of each axis was reduced in boring.