An electric device for a bicycle, the electric device comprising: a casing having an inner space; a pedal shaft mounted through the casing and having two ends protruding out of the casing ; a motor shaft mounted pivotally in the casing and mounted around the pedal shaft ; a rotor assembly mounted in the casing , connected to the motor shaft , and actuating the motor shaft ; a stator assembly mounted securely in the casing and corresponding to the rotor assembly ; a gear assembly mounted in the casing , and connected to the pedal shaft and the motor shaft , ; a chain wheel connected to the gear assembly and actuated by the gear assembly ; and a control unit mounted securely in the casing and having a circuit board electrically connected to the stator assembly and having a normal parallel to the motor shaft ; and a rotor detecting segment being adjacent to the rotor assembly and detecting the rotor assembly ; and multiple power switch devices electrically connected to the circuit board ; wherein the casing has multiple heat-dissipating fins formed on an outside surface of the casing and disposed on an outside of the power switch devices ; the electric device characterized in that : the gear assembly is actuated by the pedal shaft or the motor shaft ; and each of the power switch devices has a metal heat-dissipating surface abutting an inside surface of the casing ; a switch body parallel to the circuit board ; and multiple pins connected to the switch body and the circuit board , and bent vertically. a casing having an inner space; a pedal shaft mounted through the casing and having two ends protruding out of the casing ; a motor shaft mounted pivotally in the casing and mounted around the pedal shaft ; a rotor assembly mounted in the casing , connected to the motor shaft , and actuating the motor shaft ; a stator assembly mounted securely in the casing and corresponding to the rotor assembly ; a gear assembly mounted in the casing , and connected to the pedal shaft and the motor shaft , ; a chain wheel connected to the gear assembly and actuated by the gear assembly ; and a control unit mounted securely in the casing and having a circuit board electrically connected to the stator assembly and having a normal parallel to the motor shaft ; and a rotor detecting segment being adjacent to the rotor assembly and detecting the rotor assembly ; and multiple power switch devices electrically connected to the circuit board ; a circuit board electrically connected to the stator assembly and having a normal parallel to the motor shaft ; and a rotor detecting segment being adjacent to the rotor assembly and detecting the rotor assembly ; and a normal parallel to the motor shaft ; and a rotor detecting segment being adjacent to the rotor assembly and detecting the rotor assembly ; and multiple power switch devices electrically connected to the circuit board ; wherein the casing has multiple heat-dissipating fins formed on an outside surface of the casing and disposed on an outside of the power switch devices ; the electric device characterized in that : the gear assembly is actuated by the pedal shaft or the motor shaft ; and each of the power switch devices has a metal heat-dissipating surface abutting an inside surface of the casing ; a switch body parallel to the circuit board ; and multiple pins connected to the switch body and the circuit board , and bent vertically. the gear assembly is actuated by the pedal shaft or the motor shaft ; and each of the power switch devices has a metal heat-dissipating surface abutting an inside surface of the casing ; a switch body parallel to the circuit board ; and multiple pins connected to the switch body and the circuit board , and bent vertically. a metal heat-dissipating surface abutting an inside surface of the casing ; a switch body parallel to the circuit board ; and multiple pins connected to the switch body and the circuit board , and bent vertically. <EOS>