The air compressor is a key component in the polymer electrolyte membrane (PEM) fuel cell system and its performance has great impact on the electric power output from the system. Here, analysis was conducted firstly on the Roots-type compressor with fixed internal compression based on the volume reduction created by helical rotors. The built-in volume ratio variations with the wrap angle and the lobe numbers were calculated and discussed. Then a prototype of the Roots-type air compressor with 6 lobes was developed according to the outcomes of the research. The volumetric and isentropic efficiency of the prototype were measured under the discharge pressures from 0.11 MPa to 0.25 MPa while the rotation speeds from 10000 to 14000 rpm. Under the design conditions, the volumetric efficiency and isentropic efficiency of the Roots-type air compressor were 75.8% and 53.9% respectively. It was concluded that this kind of Roots-type air compressor is especially suitable for the fuel cell systems applied in the range extended electric vehicles.