Abstract: A surface nanocrystalline was fabricated by ultrasonic shot peening (USSP) treatment at AZ31 Mg alloy. The effect of nanocrystalline thickness and its placed side (external or internal) on the bendability was studied by a V-bending test. Three durations, 5, 10, and 15 min, were applied to form the surface nanocrystalline with thicknesses of 51, 79, and 145 μm, respectively. Two-side treatment led to a similar bendability as that of as-received. One-side internal treatment for 5 min resulted in an improved bendability while the improvement was limited and degenerated for longer treatment. The improvement was related to the drawing back of the neutral axis. The one-side external treatment also improved the bendability, and the improvement was due to the redistribution of strain and stress during bending. With nanocrystalline at external side, it resulted in a larger stress but a smaller strain at the convex, which prevented the happening of crack during bending.