Attributed to their specific atomic bonding, the soft, graphite-like, hexagonal boron nitride (h-BN) and its superhard, diamond-like, cubic polymorph (c-BN) are important technological materials with a wide range of applications(1). At high pressure and temperature, h-BN can directly transform to a hexagonal close-packed polymorph (w-BN)(2) that can be partially quenched after releasing pressure. Previous theoretical calculations(3-5) and experimental measurements (primarily on quenched samples)(6-9) provided substantial information on the transition, but left unsettled questions due to the lack of in situ characterization at high pressures. Using inelastic X-ray scattering to probe the boron and nitrogen near K-edge spectroscopy, here we report the first observation of the conversion process of boron and nitrogen sp(2)- and p-bonding to sp(3) and the directional nature of the sp(3) bonding. In combination with in situ X-ray diffraction probe, we have further clarified the structure transformation mechanism. The present archetypal example opens two enormous, element-specific, research areas on high-pressure bonding evolutions of boron and nitrogen; each of the two elements and their respective compounds have displayed a wealth of intriguing pressure-induced phenomena(10) that result from bonding changes, including metallization(11,12), superconductivity(13,14), semiconductivity(15), polymerization(16) and superhardness(2,17,18).