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Impeller is a part of centrifugal compressor that has an important role in the pressure generation process. Differences in the shape and size of this component greatly affect the overall ability of the compressor to generate pressure. This research is focused on prediction of centrifugal compressor performance by varying the impeller outlet blade angle. The study was conducted at off design parameter of a single stage centrifugal compressor with an impeller diameter of 236.24 mm and a diffuser outer diameter of 349.83 mm. The number of impeller blades is 18 and the number of diffuser blades is 6 with impeller inlet blade angle (β1) of 42°, and impeller outlet blade angle (β2) of 50°. The maximum efficiency of the designed centrifugal compressor occurs at a mass flow rate of 0.993 kg/s with a value of 83.71%. Result of the numerical simulation shows that the higher value of β2, the higher pressure-ratio that can be produced under all mass flow rate condition. Compressor overall efficiency also increases with the increase of β2 value. However, higher value of β2 results in an increase of instability level of gas flow at impeller outlet and eventually backflow began to occur in the β2 value of 90° that would reduce the volumetric efficiency of the compressor. [ABSTRACT FROM AUTHOR]