An epitaxial Li2MnO3(001) thin film electrode with layered rock-salt structure was tested in an all-solid-state battery configuration for the first time. Using amorphous Li3PO4 solid electrolyte, good discharge capacity after the 5th cycle, excellent reversibility for 100 cycles, and high rate capability at room temperature were observed. The same electrode showed significant capacity fading when cycled in liquid electrolyte. The all-solid configuration could prevent electrolyte decomposition, Mn dissolution, and oxygen extraction from Li2MnO3 electrode.