A new alumina ceramic substrate has been developed for an Electronic Control Unit (ECU). This substrate has a thick conductor circuit made of molybdenum for a power line, accepting high electric current up to 50 Amperes (standard design). The design concept can integrate control and power circuits in one substrate, directly mountable on engines or power train system without any forced cooling system. Cross-sectional shape of the conductor is designed to minimize thermal stress and reduce cracking in the substrate, caused by a thermal expansion mismatch between alumina and molybdenum. Optimization of the design is performed by FEM analyses and X-ray diffraction measurement. We confirmed that the substrate can work after 10,000 cycles of temperature cycle test, and 600,000 cycles of electric cycle test.