Abstract: The preparation method of electrolytes provided by the present invention involves applications of a first solid oxide powder and a second solid oxide powder, both of which are prepared by using a sol-gel process and a calcination process. Each of the first and second solid oxide powders is a Perovskite-type oxide. After the first and second solid oxide powders are readily mixed, they are compressed into a pellet and then sintered to prepare the afore-mentioned electrolytes for SOFC. It is found in the present invention that by mixing and compressing different solid oxide powders, the solid oxide powder having smaller particle size can fill into the gaps of the other solid oxide powder. After the pellet is sintered, the density of the product is significantly improved.