Provided is a thin film deposition apparatus. The thin film deposition apparatus includes a substrate support unit configured to support a substrate; and a shower head disposed above the substrate support unit to supply a process gas to the substrate. The shower head includes: an upper plate including a plurality of gas channels forming process gas flow paths and gas injection holes formed in the gas channels, high-frequency power being applied to the upper plate to excite the process gas into plasma; a baffle plate disposed under the upper plate and including a plurality of holes to uniformly distribute the process gas; and an injection plate disposed under the baffle plate to inject the process gas supplied through the baffle plate to a substrate.