2D materials are considered as unique class of modern materials. Understanding the mechanisms behind the exfoliation processes enables us to significantly enhance processing potential of 2D materials. In-situ synchrotron X-ray characterization is employed to study the temporal changes of hexagonal-boron nitride (h-BN) while its initial processing from bulk 3D crystalline material towards its 2D counterpart. In situ X-ray powder diffraction (XPDF) experiment is conducted with thermal cycle that heating bulk h-BN up to 1273 K and subsequent cooling. The results show a linear expansion in c-axis direction of h-BN crystals as commonly understanding, however a contraction behaviour in a-axis direction is observed up to around 750 K during heating process, followed by an expansion behaviour when temperature over 750 K. Characterization particularly indicates structural changes of long-range order favourable for exfoliation between the range of 750 K to 950 K. With the consideration of thermal oxidation studies also, a hypothesis of thermal assisted exfoliation with oxygen interstitial and substitution of nitrogen at high temperature is proposed through our studies to drive the exfoliation mechanisms.