The mechanical properties and elastic moduli of wurtzite-structured nitrides (BN, AlN, GaN and InN) and AlGaN nitride alloy are evaluated by micro- and nano-indentation studies at room temperature (RT), and the physical processes are discussed. The hardness of BN, AlN, GaN and InN varies depending on the alpha-axis lattice constant a as an (n similar to -6), whereas the Young's, shear and bulk moduli of the nitrides vary as a(n) (n similar to -5). The properties are governed by the atomic bonding. A homology of indentation hardness scaled using the shear modulus and the magnitude of the Burgers vector for the nitrides is observed. The yield strength and stress-intensity factor of the nitrides at RT are presumed. The alloy-hardening effect in AlGaN alloy is weak compared with that in InGaN alloy.