Nanoindentation behaviors of zinc oxide (ZnO) nanofilms with different film thicknesses are studied by using both molecular mechanics (MM) simulations and continuum analyses. It is found that there is a significant size effect on the indentation modulus obtained from MM simulations, which is absent in the continuum studies. The indentation modulus increases with the film thickness, and it also increases with the indentation depth; the trend of such a variation also depends on the film thickness. The contributions of the contact size effect, film thickness size effect, and microstructural size effect (surface effect) are elucidated and their couplings are explored. In addition, the substrate effect and nonlinear hyperelastic effect are incorporated to explain the size dependence of elastic indentation behaviors of ZnO nanofilms. © 2007 American Institute of Physics.