Quinacridones are industrially important red pigments characterized by NH…O intermolecular hydrogen bonds. Among these, unsubstituted γ-quinacridone (γ-QA) and 2,9-dimethylquinacridone (2,9-DMQA) are the major products used widely in painting and imaging industries. Our recent structure analysis revealed that the NH…O hydrogen bond in 2,9-DMQA is slightly weaker as compared with that of γ-QA, although the former is generally known to be more stable than the latter as to the resistance to heat and light irradiation. In order to clarify this point, the cohesion of γ-QA and 2,9-DMQA has been investigated on the basis of the energy partition analysis of semi-empirical molecular orbital calculations, using the X-ray x, y and z coordinate sets. As expected, the NH…O hydrogen bond is found to be the major intermolecular force in γ-QA and 2,9-DMQA. However, in 2,9-DMQA, an additional interaction is also operative due to diagonal pairs along the stacking axis. Then, the sum of the hydrogen bond and the diagonal-pair interactions in 2,9-DMQA is found to exceed the total cohesive force of γ-QA, resulting in a higher stability.