© 2018 The Chemical Society of Japan. CaAlSiN3:Eu2+ is a widely applied phosphor in white LEDs (w-LEDs) because of strong blue absorption and efficient red luminescence with high thermal quenching temperature. The good stability against thermal quenching has been well established, but the mechanism for the luminescence quenching at high temperatures has not been elucidated yet. In this report, we investigate the possibility of thermal ionization quenching by thermoluminescence (TL) and persistent luminescence techniques. In the TL glow curve by UV charging at 100 K, two broad TL glow bands were observed around 160 and 390 K. The higher TL glow band was not observed by 550 nm charging at 300 K, but it was observed by 550 nm charging at above 400K which corresponds to the onset temperature of luminescence quenching. Because the 550 nm light excites the lowest 5d level of Eu2+, we conclude that the luminescence quenching of CaAlSiN3:Eu2+ at high temperatures is caused by the thermal ionization.