A navigation device including an image sensor, a processor and a memory is provided. The memory stores a lookup table of a plurality of moving speeds each corresponding to one frame period. The image sensor captures image frames successively. The processor calculates a current speed according to a current image frame and a previous image frame, reads a frame period from the lookup table according to the calculated current speed, wherein the read frame period is multiplied by a ratio, which is smaller than 1, when an acceleration is confirmed by the processor according to the captured image frames.