The development of new materials with superior mechanical, electrical and magnetical properties is a major objective of the material research society. Among all the properties, hardness is one of the important properties of materials for engineering applications. In the study of hardness, by both semi-empirical formula and ab initio calculation it has been found out that large bulk modulus, which is closely related to hardness, requires short covalent bonds within a solid, and β-C_3N_4, the covalent carbononitride solid, with C substituting The development of new materials with superior mechanical, electrical and magnetical properties is a major objective of the material research society. Among all the properties, hardness is one of the important properties of materials for engineering applications. In the study of hardness, by both semi -empirical formula and ab initio calculation it has been found that that bulk bulk, which is closely related to hardness, requires short covalent bonds within a solid, and β-C_3N_4, the covalent carbononitride solid, with C-substituting