Ultrathin cross sections of core-corona polymeric nanospheres, which were prepared by the free-radical polymerization of methacrylate-terminated poly(ethylene glycol) (MA-PEG) as a hydrophilic macromonomer and styrene (St) as a hydrophobic comonomer in a ethanol/water (4/1, v/v) mixed solvent, were observed by using transmission electron microscopy (TEM). Conditions of the preparation of nanosphere-embedded resins, the cutting rate by a microtome for cross-section preparation, and staining with osmium tetraoxide were optimized in order to analyze cross-sectional morphologies of the nanospheres clearly. Nanospheres prepared showed a core-corona structure in all cases. The corona laver thickness was constant at around 6 nm when we used the same macromonomer (M-n 996), even if we altered the size in the nanosphere to 133, 272, and 1530 nm by altering the monomer ratio (St/MA-PEG 20/1, 50/1, and 200/1, respectively, mol/mol). The thickness increased from 6.7 to 17 nm with an increase in the molecular weight of a macromonomer from M-n 996 to 4250, although the size decreased from 373 to 204 nm with an increase in it. TEM observations in the present study led us to have further structural understanding of core-corona nanospheres that had various technological and biomedical applications.