Nanoindentation hardness tests were conducted on various Al x Ga 1-x N alloys grown by metal-organic vaporphase epitaxy. AlGaN alloys showed the hardness of 16.5−19.5 GPa strongly dependent on the alloy composition with a maximum at x~0.5, while the Young's modulus was ~ 300GPa, almost constant in the whole composition range investigated. The observed alloy hardening was confirmed under considering the effects of residual in-plane strain and threading dislocation densities.