A new vacancy model by using periodic density functional theory was used to describe the process of In segregation from clean Al surfaces via atomic movement through vacancies. The detailed segregation mechanism of impurity metal In to Al (001) surface planes is examined, carefully evaluating energy barriers for each step in the segregation process. The results show that the system energy is decreased by 0.46 eV and the highest energy barrier is 0.34 eV when the impurity atom In moves from the second layer to the topmost layer in the Al (001) slab. The system energy is almost constant when impurity atoms segregate from the third layer to the second layer. Higher energy (0.65 eV) was needed to overcome the energy barriers. So, In showing a strong segregation to the clean Al (001) surface is thermodynamically favorable.