Oxyriitride glasses areni-iixed-anion systems, in whiCh the 2-fold coordinated oxygen atoms have, been partially substituted by 3-fold coordinated nitrogen atoms. This so-called nitridation pros intruchices additional 'bonds and thereby constrains and compacts the glass network and consequently alters the glass, hardness. To explore how and why hardness varies With the degree of nitridation, we have derived atopological model of oxynitride glass hardness iflg temperature-dependent coristraint theory, by which the scaling of glass hardness with nitrogen Content can be predicted. A linear model has been derived based on the assuinption that the substitution of oxygen atoms with nitrogen atoms is responsible for the hardness increase due to the increase in the number (n) of bond-bending and bond-angular constraints. It turns out that the model agrees with the experimental observation, i.e., an approximate positive linear trend of the hardness change with nitrogen content-is observed for a wide range of glass compositions. The topological model may thus be useful for designing new oxynittide glass compositions with targeted 'hardness values.