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 first computes a set of match values for possible motion vectors of a sample image with respect to a reference image and applies a filtering operation to compute a respective smooth coefficient for each match value. The method further computes a locally minimum number in the set of match values according to a first rule, such that the sample image is regarded as a qualified image when the locally minimum number is smaller than a threshold and accordingly a global minimum in the set of match values is found as a motion vector of the sample image.