A thin film deposition apparatus, which has high external light transmittance and a cathode having a uniform thickness, is disclosed. In one embodiment, the thin film deposition apparatus includes a deposition source spaced apart from a center of the substrate and configured to emit a deposition material toward the substrate and a blocking member disposed on the substrate and configured to block at least part of the deposition material emitted from the deposition source, wherein an opening is defined on at least one side of the blocking member.