Assembled morphologies of nanoparticles with controllable interfaces are critical to provide tailorable polymer nanocomposites (PNCs) with unique properties. The active assembly of nanoparticles using an external magnetic field is a potential solution to the bulk fabrication of PNCs due to the low power requirements, ease of use, high periodicity, and patterning capability. However, additional information is needed to produce controllable, and homogeneous nanoparticle assemblies within a polymer matrix. This work is a continuation of experimental and theoretical work about the assembly of iron oxide nanoparticles in static and oscillating magnetic fields.28,30 Previous work included evaluation of critical manufacturing parameters and their ranges on the assembly of iron oxide nanoparticles in a low viscosity matrix. Based on this previous work, magnetic assembly in a high viscosity matrix (polymer) was studied. Our results demonstrate that effective assembly of iron oxide nanoparticles (~25 nm γ-Fe2O3, 0.1% volume fraction) can be accomplished in highly viscous matrices at very low magnetic flux densities (50 G DC). In addition, a magnetic field gradient across the sample domain of over 5% has been shown to produce significant translation of the nanoparticles towards the pole locations, limiting control of the particle assemblies. The usage of a silane coupling agent (GPS) has also been shown to be effective in controlling settling of the nanoparticles, leading to dispersed and homogenous nanoparticle assemblies. Lastly, while a higher nanoparticle concentration (1.0% volume fraction) produced additional settling, nanoparticle assemblies were found to be uniformly distributed throughout the polymer matrix similar to that found for the silane coupling agent modified nanoparticles. The understanding of how to produce homogeneous and uniform nanoparticle assemblies within a high viscosity matrix is important prior to demonstrating the effective tailorability of nanoparticle assemblies in PNCs, and their influence on transport properties. In the near-future, fabrication of homogeneous PNCs with variable transport properties will be demonstrated.