Hardness of the pseudobinary transition metal aluminum nitride (T-Al-N) films is improved with increasing the AlN content as far as the B1structure is maintained. A drastic change in the compositional dependence of the hardness corresponds to the phase change of the pseudobinary nitride from B1(NaCl) to B4(wurtzite) structure. Predicted value of AlN content for the drastic change agrees with the AlN content determined experimentally. Hardness of various T-Al-N films was closely correlated with the bulk modulus calculated from interatomic distance based on the power functional formula. The improvement of hardness is attributed to the inherent increase of bulk modulus due to dissolution of AlN into transition metal nitride.