In order to establish the design methodology of an ultra micro centrifugal compressor, which is the most important component of an ultra micro gas turbine unit, a 10 times size of the final target compressor (impeller outer diameter 40 mm, corrected rotational speed 220,000 r/min) was designed. The problems to be solved for downsizing were examined and a 2-dimensional impeller was chosen as the first model due to its productivity. The conventional 1D prediction method and CFD were used. The prototyped compressor was tested by using cold air at the reduced speed of 110,000 r/min. Following to the 10 times model, a 5 times size of the final target model having fully 3-dimensional shape (impeller outer diameter 20ram, corrected rotational speed 500,000 r/min) was designed and tested by using hot gas at the reduced speed of 250,000 r/rain.