In order to understand precisely the structure and properties of interface in solids, it is important to elucidate the disordered atomic configuration on the basis of quantum mechanics. In this study, the structure and energy of Σ9 tilt grain boundary in aluminum is analyzed by ab initio molecular dynamics simulation based on the density functional theory. The results obtained are summarized as follows. (1) The grain boundary has a pair of atoms which are close to each other, and one of them moves along the grain boundary during relaxation. (2) The maximum valence charge density near the grain boundary is 0.03 a.u.^-3, which is nearly equal to that in single crystal. (3) The grain boundary energy is 0.431 J/m², which is smaller than that of Σ5 tilt grain boundary, 0.458 J/m². Moreover, the validity of Morse potential and EMT (Effective Medium Theory) potential is examined by comparing the atomic force and energy with those obtained in ab initio simulation. (1) The grain boundary structure and the force acting on each atom evaluated by EMT are similar to those in ab initio simulation. However, the grain boundary energy obtained by EMT potential is much smaller than that obtained by ab initio simulation. (2) Morse potential does not give adequate values of atomic force as well as grain boundary energy.