In this study, the size dependent hardness of silver nanowires with a five-fold twin structure was examined using nanoindentation. As the diameter of the nanowires is reduced, the five-fold twin boundaries restrict the dislocation motion, and therefore a size dependent plasticity is expected for these uniquely structured nanowires. The polyol reduction method with modifications was used to synthesize silver nanowires with different diameters in the range of 70 nm to 144 nm. The nanoindentation experiments were performed on silver nanowires deposited on a stiff MgO substrate, and the resulting h, P, and S data were analyzed using the analytical double contact model for nanowire indentation. The hardness of the nanowires determined using the double contact model showed an increase in the hardness with reduction in the diameter of the nanowires, as expected due to the presence of the twin boundaries. The hardness values determined using the analytical double contact model compared favorably to the hardness values calculated from the contact areas that were extracted from finite element method simulations of an elastic indentation into the silver nanowires on the MgO substrate.