The effect of nitrogen substitution on the crystallisation of an oxynitride glass in the Y-Ca-Si-Al-O-N system has been studied. The appropriate heat treatment temperatures were selected according to the information provided by the differential scanning calorimeter (DSC) measurement. There is a significant increase in T g and T c with increasing nitrogen content. X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis demonstrated that, for the oxide glass and oxynitride glasses containing 6 equiv.% and 12 equiv.% nitrogen, crystallisation results in the formation of irregular lath-shaped Ca 4 Y 6 O(SiO 4 ) 6 and stick-shaped anorthite. As the nitrogen content increases to 18 or 24 equiv.%, irregular plate-like yttrium-aluminium garnet (YAG) is identified as the main crystalline phase. As the nitrogen content increases to 30 equiv.%, microscopic needle-like crystals of Al 6 O 3 N 4 become the only crystallised phase. The best composition, owing to the mechanical properties (e.g. flexural strength of 162MPa and Vickers hardness of 8.5GPa), was found to correspond to a nitrogen content of 24 equiv.%.