We report a detailed theoretical investigation of the phase transition of ammonia borane (NH3BH3),from a tetragonal I4mm phase with disordered orientation of hydrogen to an orthorhombic phase with Pmn21 symmetry, as a function of temperature based on Density Functional Theory calculations with a semi-empirical dispersion potential correction.We redefine a series of sub-structures in which the NH3BH3 molecules always keep a C3v symmetry, and the partially occupied high temperature state can be described as a continuously transformation between these sub-structures.To understand the role of the van de Waals corrections to the physical properties, we employ the empirical Grimmes dispersion potential correction (PBE-D2).Both PBE and PBE-D2 functional yield almost the same energy sequence along the transition path, however, PBE-D2 functional expresses an obvious advantage in describing the lattice parameters of ammonia borane that play a major role in the unit cell shape and size.With the total energy surface along the rotation of the-[NH3] and-[BH3] groups in the solid, respectively, all possible transformation paths are mapped out.