An effective casing treatment was developed in order to satisfy the requirement of the wide flow range for turbocharger compressors. The surge margin of a compressor was expanded by the casing treatment with the inclined vanes in the cavity. The surge flow rate was 20% smaller than that of the conventional casing treatment at the pressure ratio of 3.0. The flow measurements and CFD analysis in the impeller with the casing treatment were carried out. At the low flow rate, the reversed flow through the cavity with the inclined vanes developed the counter swirl flow at the impeller inlet. The counter swirl flow caused an increase of the work coefficient. Consequently, the recirculation flow was increased as the compressor fl ow rate was reduced and the surge margin was improved significantly.