Nanostructures of sputtered thin films consisting of cobalt oxide and soda lime glass were analyzed. Transmission electron microscope showed the thin film including 9.0 mol.% Co3O4 had nanosize lamellar structure consisting of two amorphous phases. The thin film containing 82.5 mol.% Co3O4 consisted of nanosize columned Co3O4 particles with an amorphous grain boundary phase. Average grain size was 10.3 nm and standard deviation of the grain size was 1.97 nm. A relation between nanostructure and composition of the films, and a mechanism of formation of the nanostructures were discussed in terms of solvability of cobalt ion in the glass matrix.