1. A method of processing data of an optical mouse, comprising: calculating a motion vector of the optical mouse through processing of digitally converted image pixel data in an Image Signal Processor (ISP) by sequentially overlapping a reference image of an n−1 th frame to a present image of an nth frame and determining the most approximately equal parts in the reference and present images as a moving direction of the optical mouse; and transmitting the calculated motion vector to a personal computer through an interface; wherein the ISP includes an X axis navigation engine for processing data on a reference image for the motion of an X axis direction among the processed image pixel data and a Y axis navigation engine for processing data on a reference image for the motion of a Y axis direction among the processed image pixel data, and the X and Y axis navigation engines each store the reference image of the n−1 th frame, the present image of the nth frame, and a reference image of the nth frame.