In this study, we investigated the effect of viscoelastic behavior on contact stiffness value at the onset of unloading using nanoindentation experiments for two thermoplastics: poly (methyl methacrylate) (PMMA) and polycarbonate (PC). To achieve this, we employed four different load functions at two constant strain rates (0.5 and 1.0 /s), with or without holding, and the rate of unloading segment (slow or fast). Our results indicate that the contact stiffness values exhibit negative values up to 10 and 3.5% of unloading for PMMA and PC, respectively, due to viscoelastic effects, while the load function includes no holding and a slow unloading rate. However, incorporating a prolonged holding and fast unloading rate (25 times) load function significantly reduces the viscoelastic effect. Furthermore, a comparison of nominal and experimental contact stiffness values revealed divergences influenced by factors such as the implemented load function, maximum applied load, the selection of unload data fitting ranges, and the viscoelastic nature of the material.