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This paper describes the magnetic circuit design of a coaxial AC motor system, comprising one stator and two rotors, and the test results obtained for a prototype motor. The rotors of the motor share the same stator core and coils, and each rotor uses its magnetic part as a yoke. Magnetic flux linkage of each rotor was determined in consideration of the maximum torque/power conditions and maximum motor speed. Finite Element Method were utilized to design a magnetic circuit for achieving the magnetic flux linkage specification. Tests conducted with a prototype motor showed that the torque characteristics can be divided into magnetic torque and reluctance torque, just like an ordinary IPM motor. Each torque level was improved through field-weakening control. The combined torque obtained when the two rotors were driven simultaneously approximately equaled the sum of the individual torques when the rotors were driven independently.