1. An optical navigation method, comprising: sequentially obtaining plural images including a first, a second, and a third image; choosing a main reference block in the first image according to a predetermined sampling region and sampling size; comparing the main reference block and the second image by block matching comparison to determine a first motion vector; resizing the main reference block according to the first motion vector to generate an ancillary reference block having a size smaller than the main reference block; and comparing the ancillary reference block and the third image by block matching comparison to determine a second motion vector.