In order to make clear the relation between the efficiency and the internal flow condition of a centrifugal compressor, the theoretical analysis and the experiments were performed using three kinds of impellers and fifteen kinds of vaned diffusers. The boundary layer analysis, in which the cross flow is considered approximately, was carried out. The low energy flow on the shroud side and the blade suction side of the impeller was indicated both from the calculation and the experiment. The static pressure recovery coefficient of vaned diffusers and the optimum flow incidence angle were investigated. This paper describes the calculation and the experiment results.