To understand the Leidenfrost phenomenon, which is the results of formation of a thin vapor layer, the progression of the vapor should be analyzed. However, due to the limitation of measuring techniques, the empirical measurement of the vapor layer under a dynamic Leidenfrost drop as a function of time has not been reported because the vapor is only tens of micrometers thick and forms within a tenth of a millisecond. Therefore, this paper presents a synchrotron X-ray imaging with the precise resolution to overcome the limitation of previous measurement technique. The liquid vapor interfacial behavior of a drop of ethanol that is being levitated above a flat SiO2 surface by the Leidenfrost phenomenon is analyzed depending on surface temperature. Measurements suggest that a thin (<2 mu m) vapor layer develops between the surface and the drop; i.e. that the liquid does not contact the solid. The measured thickness of this vapor layer under a dynamic Leidenfrost drop was less than the thickness of the vapor layer estimated by analytical solution of a model of vapor layer thickness for a static Leidenfrost drop. The new technique presented in this study will support transient numerical simulations or an analytical solution of the vapor layer under a dynamic Leidenfrost drop, and may have applications in research on the effects of artificial surface structure on the Leidenfrost phenomenon. (C) 2018 Elsevier Ltd. All rights reserved.