A rotor orientation measurement system based on photoelectric encoder which can be applied to permanent magnet（ PM） spherical motor of a three degrees of freedom（ DoF） is designed. Structure of the PM spherical motor and the description method of spherical orientation are introduced. By analyzing differential motion

error matrix of the measurement system is deduced

and the performance of the system is analyzed. In this system

after frequency doubling

phase discrimination

reversible counting and storage with the hardware circuit

the output pulse of photoelectric encoder is calculated by microcontroller unit（ MCU）. Measured rotor orientation is displayed on LCD in realtime

and is sent to closed-loop control system of motor through serial port. Validity of the measuring system is verified by experiment.