All-solid-state batteries have been proposed as a solution to the risk of ignition and degradation of liquid electrolytes. Unlike liquid electrolytes, solid electrolytes are more influenced by their microstructure in the movement of lithium ions. In this study, we investigated how changes in the geometry of the microstructure lead to changes in battery performance. Three-dimensional microstructures were created by randomly arranging active material (AM) and solid electrolyte (SE) particles of different sizes. Microstructures with constant-sized SE particles showed that the contact area between AM and SE particles decreased as the size of the AM particles increased. The microstructure geometry factors, such as the volume ratio, porosity, and contact area of the AM and SE particles, were applied to the battery model. The simulation results showed that the microstructure geometry did indeed affect the battery performance.