Nanoindentation hardness tests were conducted on various AlxGa1-xN alloys grown by metal-organic vapor-phase epitaxy. AlGaN alloys showed the hardness of 16.5-19.5 GPa strongly dependent on the alloy composition with a maximum at x similar to 0.5, while the Young's modulus was similar to 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.