A two‐stage small Stirling cooler has been developed and tested for the infrared astronomical satellite ASTRO‐F that is planned to be launched by Japanese M‐V rocket in 2005. ASTRO‐F has a hybrid cryogenic system that is a combination of superfluid liquid helium (HeII) and two‐stage Stirling coolers. The mechanical cooler has a two‐stage displacer driven by a linear motor in a cold head and a new linear‐ball‐bearing system for the piston‐supporting structure in a compressor. The linear‐ball‐bearing supporting system achieves the piston clearance seal, the long piston‐stroke operation and the low frequency operation. The typical cooling power is 200 mW at 20 K and the total input power to the compressor and the cold head is below 90 W without driver electronics. The engineering, the prototype and the flight models of the cooler have been fabricated and evaluated to verify the capability for ASTRO‐F. This paper describes the design of the cooler and the results from verification tests including cooler perfo...