Cellulose nanofibrils (CNFs) are promising candidates as a sustainable polymer reinforcement material, but incompatibility makes it difficult to be incorporated into a polymeric matrix. Pickering emulsion-templated synthesis, in which nanometer-sized polymeric particles covered with CNFs are formed, provides a useful route to disperse nano-sized fillers within incompatible polymers. However, CNF does not always adsorb to monomer droplets (e.g., methyl methacrylate), which is the most important step fabricating polymeric particles covered by CNFs. Here, by adding an appropriate oil to a monomer, we show that the adsorption of CNF on the oil can be dramatically enhanced by controlling the interfacial tension. Using this approach, poly (methyl methacrylate) (PMMA) nanocomposites reinforced with CNFs were fabricated. The PMMA nanocomposite films exhibit high optical properties and mechanical properties as well as thermal stability. The Pickering emulsion template with the additional oil approach can be applied to any emulsion-based polymerization methods for nanocomposite materials.