Fracture-resistant gallium nitride substrate, and methods of testing for and manufacturing such substrates are made available. A gallium nitride substrate (10) is provided with a front side (12) polished to a mirrorlike finish, a back side (14) on the substrate side that is the opposite of the front side (12). A damaged layer (16) whose thickness d is 30 .mu.m or less is formed on the back side (14). Given that the strength of the front side (12) is I.sub.1 and the strength of the back side (14) is I.sub.2, then the ratio I.sub.2/I.sub.1 is 0.46 or more.