Abstract We measured temperature dependence of resistivity under high pressure for oxygen-annealed FeTe 0.8S 0.2 using piston cylinder cell. With increasing pressure, the zero resistivity temperature ( T c zero ) decreased. Although T c zero was suppressed, intrinsic T c onset gradually increased with increasing pressure. The T c onset was estimated to be ∼13 K at 1.86 GPa from the difference of resistivity caused by suppression of T c onset under magnetic field. This result is different from the result of air exposed FeTe 0.8S 0.2. The increase of intrinsic T c onset implies that local area where the T c increases with increasing pressure exists. If FeTe 1− x S x with less excess-Fe concentration could be synthesized, FeTe 1− x S x might show positive pressure effect.