The equilibrium geometries and electronic properties of Aun Al, up to n = 13, have been systematically investigated using the density functional theory. The results show that, for the AunAl clusters, two pattes are identified. Patte one (n = 2, 3, 8), the lowest-energy geometries prefer two-dimensional structures. Patte two (n = 4-7, 9-13), the lowest-energy geometries prefer three-dimensional structures. According to the analysis of the binding energy and the fragmentation energy, AunAl clusters with odd n are found to be more stable than those with even n. The same trend of alteation can be illuminated according to the computational results in the HOMO-LUMO gap, the ionization potential, and the electron affinities. The Al atom not only changes the structures of pure gold clusters, but also enhances their stabilities. NBO analysis indicates 6s orbital of Au atom hybridizes with 3p orbital of Al atom.