Concerning the conceptual design of an ultra micro gas turbine, a compressor with the impeller outer diameter of D₂=4mm was investigated at MIT for the feasibility study of an ultra micro gas turbine including manufacturing process. Because the ultra micro centrifugal compressor is one of the most important component of an ultra micro gas turbine unit, a 5 times and a 10 times sizes (D₂=20mm and 40mm) for the final target were prototyped and tested in the present study. The similitude for these impellers including the relationship between the rotational speed and the pressure ratio was examined for the experimental results of the 5 times and the 10 times models. As a result, the piping layout downstream of the compressor was proved to affect the performance characteristics of the compressor. The total loss of 5 times model was much larger than that of 10 times model due to the scale effect.