Dense and thin electrolytes composed of yttrium-stabilized zirconia for anode-supported solid oxide fuel cells were produced by slip casting of different powder suspensions. These suspensions were obtained by mixing different proportions of nanocrystalline powder made by spray pyrolysis and commercial micro-sized powder in ethanol. Two parameters influence mostly the formation of gastight thin films: the powder morphology which controls the green density of the films and the powder sintering activity on which depends the formation of cracks during drying and sintering and adhesion. Here, optimized powder mixtures allowed the formation of a 5 μm thick electrolyte with efficient gastightness besides industrial applications. It is shown in this study that the optimum suspension composition corresponds to the best compromise between powder morphology and powder sintering activity.