Recently, core-shell type spherical nanoparticles (nanospheres) (core: cerium oxide, shell: organic polymer) that have a mono-disperse size distribution and disperse easily in water and alcohol have been reported. Core-shell type cerium oxide/organic polymer nanospheres were characterized with respect to physicochemical properties and microstructure. As-prepared nanospheres have no pores with a size of 4 nm or less, while the nanospheres calcined at 600°C had the pores with a size of 4 nm or less. These pores are considered to be on the surface of the core. The as-prepared nanospheres and those calcined at 600°C had size distribution peaks in the range of 20-60 and 70-110 nm, respectively. The specific surface area and total pore volume of the as-prepared nanospheres were 82.8 m2/g and 0.211 cm 3/g, respectively. For core-shell type nanospheres with a size of 114 nm, the diameter of the core particles was 88-92 nm and the thickness of the shell was 11-13 nm. Transmission electron microscopy images revealed that the core had dense structure. The average density of the core-shell type nanospheres was in the range of 4.2-4.6 g/cm3.