The formation mechanism of how graphite can be transformed into diamond at ultra high-pressure and-high pressure (UHPHT) has been under debate for decades. By using the empirical electron theory of solid and molecules (EET), the valence electron structures of graphite and diamond are constructed for the conditions of diamond synthesis by the static pressure and detonation methods, respectively. The relative differences in electron density were subsequently obtained for the interfaces of 12 different combinations of crystal planes of graphite and diamond at UHPHT. It is found that these relative differences in the static pressure condition are much greater than 10%, indicating that the differences of valence electron structures at the graphite/diamond interfaces are too large to induce the direct transformation of graphite into diamond. Moreover, the difference between experimental and theoretical bond lengths in the graphite structure for the detonation synthesis condition is 0.1073 nm, being significantly greater than the maximum value (0.005 nm) required for the stable valence electron structure of a crystal. This means that the valence electron structure of graphite under the detonation synthesis condition is very unstable. Therefore, we conclude that graphite might have to firstly decompose into a metastable phase before it can be transformed into diamond at UHPHT.