Polymer nanocomposites (PNCs) are important in various engineering fields due to their unique combination of properties. For industrial applications, an accurate knowledge of the mechanical properties around filler particles is crucial. However, the physical behavior of PNCs cannot be predicted by the simple convolution of the matrix and filler bulk properties due to the presence of an interphase, i.e., a region where material properties are significantly altered due to the presence of a nearby surface, typically up to 100 nm in extent. The interphase thus plays a critical role in the mechanical properties of a PNC, especially when interphase forms a relatively large volume fraction. Recent studies have shown that the interphase exhibits a gradient of mechanical properties due to the complex details of the matrix mechanical properties in the vicinity of the filler surface. Thus, detailed local characterization of the interphase mechanical properties is necessary to understand the overall mechanical properties of PNCs and thus their performance for a broad range of applications.