The collapsing layer structure of hydrogen phosphate intercalated Mg/Al-layered double hydroxides (LDH-HPO4) was investigated by high temperature in-situ X-ray diffraction with synchrotron radiation (in-situ XRD). The thermal behavior of the layer structure of LDH-HPO4 showed the same behavior in both in-situ and ex-situ XRD up until 109 °C as previously reported. The basal spacing of LDH-HPO4 continuously decreases above this temperature and finally the layer structure of LDH-HPO4 collapses. Another diffraction peak, d110, which gives a lattice constant a0, decreases at 62 °C, keeps constant to 120°C, increases up to 176°C and again keeps constant until the layer collapse. In this study, the different thermal behavior is observed between two reflections by in-situ XRD measurement, which explains the mechanism of the collapse of the layer structure in LDH-HPO4.