Solid oxide fuel cells (SOFCs) with high reaction activity to use the hydrocarbon fuels are promising for solving environmental pollution problems. We designed and evaluated an SOFC system operated at 800degreesC, which uses cells and heat exchangers of micro-tubes with the outer-diameter of 2.4 mm. We found that a compact SOFC system (55.8 kg, 0.064 ml) can supply a maximum power of 64 kW for driving and that the system efficiency is 3-4 times higher than an internal combustion engine (ICE) system with a power output less than 20 kW.