Atomic Force Microscopy (AFM) was shown to be the powerful tool for nano-scale characterization of a fracture surface. The fatigue striations of 2017-T351 aluminum alloy were observed using AFM, and from their cross sectional profile of the three-dimensional images, widths and heights of the striations were analyzed. The following results that will be helpful to understand the fatigue crack growth mechanism were obtained. (1) Crack growth rate was the same as a striation width down to the range of 10-5 mm/cycle. (2) The striation height increased with the striation width. (3) A ratio of the striation height to its width increased as the stress intensity factor range increased. (4) Effect of a precipitate on the morphology of striation was changed by the relative dimensional difference between the striation width and the precipitate. (5) Both striation height and its width showed a power law relation with a crack tip opening displacement (CTOD).