Using a first-principles total energy method based on density functional theory within local density approximation, we have performed a first-principles computational tensile test (FPCTT) on an Al grain boundary. The theoretical tensile strength of the Al grain boundary is calculated to be 9.5 GPa at the strain of 16%. Based on the valence charge density, bond length and atomic configuration evolution with increasing strain, we demonstrate that the fracture occurs at the grain boundary interface，characterized by breaking of the grain boundary interfacial-reconstructed bonds. We further found that, because of the reduced number of the nearest neighbor of Al atoms in the grain boundary, these interfacial-reconstructed bonds have similar features to covalent bonds, which results in rather high grain boundary strength in comparison with the Al bulk.