A mobile device including an acceleration sensor, a magneto sensor, a gyroscope sensor, and a processor is provided. The acceleration sensor, the magneto sensor, and the gyroscope sensor generate acceleration data, magnetic field data, and angular velocity data, respectively. The processor determines a pitch angle and a roll angle according to the acceleration data, determines a yaw angle according to the magnetic field data, and determines a quaternion according to the pitch angle, the roll angle, and the yaw angle. Also, the processor determines orientation information of the mobile device according to the quaternion and the angular velocity data, and adjusts the orientation information according to the acceleration data and the magnetic field data.