LiMgPO₄ based optically stimulated luminescence (OSL) phosphors are found to have good dosimetric characteristics. Thermal quenching is the decrease in luminescence efficiency of the phosphor at high temperature. In this paper we report the thermal quenching behaviour of LiMgPO₄:Tb, LiMgPO₄:Tb,B and LiMgPO₄:B (LMP-N) OSL phosphors. The quenching has been investigated by recording the thermoluminescence (TL) glow curves with increasing heating rates. To have some knowledge about the centres responsible for quenching the luminescence was recorded at selective wavelengths. The results suggest that the centres associated with B are responsible for the quenching.