Trimer is the smallest cluster that can have a one-dimensional or a two-dimensional structure on fcc (111) surface. Using first-principles density-functional-theory calculations, the structural and dynamical properties of Al trimer on Al(111) surface have been studied in detail. Al trimer on Al(111) surface has four close-packed (compact) triangular configurations, two linear configurations, and some other noncompact triangular configurations. The close-packed triangular trimers are more stable than the noncompact triangular trimers as well as the linear trimers. For the dynamics of Al trimer on Al(111) surface, the diffusion processes are much more complicated than the adatom and dimer diffusions. There are three different kinds of diffusion mechanisms: concerted translations and rotation of compact triangular trimers (the energy barrier, Ed=0.24 eV); back and forth transformation between compact triangular trimers and linear trimers (Ed=0.21 eV); and translation of linear trimers (Ed= 0.28 eV). Among these different mechanisms with similar height of diffusion barriers, the concerted translations of the compact triangular trimers have the longest displacement of the center of mass in each step. Therefore, we expect that the long-range diffusion of Al trimer on Al(111) surface is dominated by the concerted motion process of the compact triangular trimers.