Abstract: C¹ natural element method for couple-stress (CS) theories was constructed when C¹ natural element method was applied to CS theories. Considering the fact that the thickness of film is on the micron scale, which is close to the material characteristic length scales, the size effect of interfacial shear stress between film and substrate was studied. Numerical results illustrated that with decrease in the film thickness, normalized interfacial shear stress (defined as the ratio of the shear stress in the CS theories to that in the classical theories) decreased gradually, while the size effect became stronger gradually. For a given bonding strength, the shear stress in the CS theories was less than that in the classical theories, which implied that it is hard to debond in the CS theories.