1. A method for immediately determining incorrect pixel values of a captured image on an optical mouse, which determines a quality of an image in a video stream captured by the optical mouse and accordingly finds a motion distance for the optical mouse, the method comprising the steps: a reference image capturing step, which captures an image as a reference image from the video stream; a sample image capturing step, which captures an image after the reference image for use as a sample image; a match value computing step, which computes a set of match values for possible motion vectors of the sample image with respect to the reference image; a smooth coefficient computing step, which applies a filtering operation to compute a respective smooth coefficient for each match value; a locally minimum number computing step, which computes a locally minimum number in the set of match values according to a first rule; and an image quality determining step, which determines if the locally minimum number is smaller than a first threshold; if yes, the sample image is a qualified image and thus a global minimum in the set of match values is found as a motion vector of the sample image.