In this study, control algorithm, based on fuzzy logic, for compressor speed and opening percentage of electronic expansion valve (EEV), was investigated. The main aim of this study is to increase efficiency of the chiller system, having variable speed scroll compressor (VSSC) and EEV. Current chiller system has an air cooled condenser, a shell-tube liquid evaporator, and a vertical scroll compressor. Flow of the refrigerant R134a was controlled by EEV in the system. Chiller systems are designed for working at maximum load conditions; on the contrary, these plants work under partial load conditions most of their life cycle by means of thermostatic control. Refrigeration capacity modulation methods, such as variable speed compressors, allow to match continuously the compressor refrigeration capacity to the load, determining an energy saving according to classical thermostatic control. In this study, compressor speed was controlled with fuzzy logic algorithm according to chiller water output temperature and EEV opening percentage was controlled with fuzzy logic algorithm according to refrigerant superheat value at outlet of the evaporator. Compressor speed has been changed between 30 and 60 Hz and EEV opening percentage has been changed between 10 and 45% of the maximum opening percentage. In fact, the chiller plant was operated as on/off at 50 Hz fixed speed by a thermostat. During the study, it has been operated as variable speed by adding PWM inverter. In the present study, fuzzy logic controlled variable speed chiller system exhibited 17% coefficient of performance increase according to the thermostatic controlled fixed speed system. Also, better water temperature control was obtained via fuzzy controlled VSSC. Copyright (C) 2009 John Wiley & Sons, Ltd.