The key step in the sol-gel processing of thin films is the removal of cracking in the drying and calcining stage. We could successfully balance the film formation property and the mechanical toughness to cracking by mixing polymeric and particulate sols of the same material. Crack-free YSZ thin membranes were, thus, prepared on a porous support. In this paper, the detailed mechanism of the film toughening is studied. Small angle X-ray scattering of the sols and nitrogen adsorption/desorption of the calcined oxides were carried out, which corresponded well to each other. Based on these results and the FE-SEM observations of the supported thin film, we propose a networking model of spherical particle agglomerate by linear inorganic polymer to interpret the formation mechanism while keeping the mechanical toughness. This approach should be applicable to other metal oxides and mixed oxides systems.