Abstract: Al-Cu alloy films with copper atomic fractions of 0-11.8% were prepared on stainless steel by magnetron sputtering method. The effects of copper content on the microstructure, nanoindentation mechanical properties and strengthening mechanism of the films were investigated. The results show that the pure aluminum film had a face-centered cubic structure. When the copper atomic fraction was between 2.2% and 6.5%, supersaturated solid solution phases were formed in Al-Cu alloy films. When the copper atomic fraction exceeded 6.5%, AlCu compounds were formed in films. With the increase of copper content, the grain size of the film decreased, and the hardness and elastic modulus increased. When the copper atomic fraction increased to 11.8%, the grain size was 34.7 nm, and the hardness and elastic modulus increased by 212.5% and 2.2% those of the pure aluminum film, respectively. When the copper atomic fraction was between 0 and 6.5%, the strengthening of films mainly depended on grain refinement strengthening and solid solution strengthening. When the copper atomic fraction exceeded 6.5%, the strengthening of films was determined by the combination of grain refinement strengthening, solid solution strengthening and second phase strengthening.