The precise mechanical property evaluation of a solid electrolyte interface (SEI) is crucial for the mechanical stability of the SEI on silicon anodes, which significantly expand during lithiation. Herein, cyclic loading tests and numerical methods were used to quantify the elastoplasticity and viscoelasticity of SEIs and the effects of the stress field in a silicon anode material below the SEI to precisely evaluate the elastic modulus of the SEI. Instrumented nanoindentation was used to quantitatively observe the indentation force response on the heterogeneous surface of composite electrodes. The mechanical properties of the SEI with a nanothin film structure probed by a simple analytical model assuming perfect elastic and homogeneous mechanical properties in a domain subjected to the indentation force resulted in significant elastic modulus overestimation. The substrate effect was significant, especially for reduced SEI thickness, and could cause an "apparently"mechanically bilayered structure of the SEI with a hard inorganic inner layer and a soft outer polymeric layer. The elastic modulus of the SEI formed in fluoroethylene carbonate (FEC) electrolyte (3.7 GPa) agreed with that previously predicted from the mechanical properties of cross-linked polymers in the SEI formed from a FEC electrolyte and supported the recent solid-state NMR results reporting the existence of polymeric species on the interfacial region of the FEC-SEI and the silicon anode.