This report introduces modified cellulose polymers as new emulsifiers of cubosomes. We prepared novel nanoparticles containing cubic-phase-forming lipids using hydroxypropyl methylcellulose acetate succinate (HPMCAS). Small-angle X-ray scattering showed a much lower incorporation of HPMCAS into the cubic structure of monoolein than did a conventional emulsifier, Pluronic F127, which is known to modify the cubic structure. Cubosomes prepared with HPMCAS showed roughly equal stability as nanoparticles with Pluronic F127. These results suggest that HPMCAS can be a novel emulsifier of cubosomes, which brings about no internal structure modification.