Abstract The surface eigenstress model was further developed to study the surface-induced size-dependent ultimate biaxial tensile strength of thin films. First-principles calculations and molecular dynamics (MD) simulations were conducted on Au and Si thin films with different thicknesses. The atomistic calculations show that the ultimate biaxial tensile strength of Au thin films increases when the film thickness decreases, a typical the thinner the stronger phenomenon, while the Si thin films exhibit the thinner the weaker behavior. Nevertheless, both behaviors were perfectly predicted by a nonlinear scaling law developed from the surface eigenstress model. Moreover, the first-principles calculations put insight into the surface induced strengthening or weakening mechanism.