A portable electronic device includes a housing, a rotation sensor, and a processor. The rotation sensor includes a supporting plate fixed to the housing, a cylindric carrier, an electron emitting device, a magnet, a number of strip-shaped electrodes, and a detecting device. The cylindric carrier is disposed on the supporting plate and is rotatable relative to the supporting plate. The electron emitting device is disposed on the supporting plate and emits electrons along the center axis of the cylindric carrier. The magnet provides a magnetic field substantially parallel to the supporting plate. The detecting device detects the falling position of the electrons and generates a signal representing the parameters of the rotation of the portable electronic device. The processor receives the signal generated by the detecting device and generates an operating signal for operating the portable electronic device.