A series of compact La/O-vacant La1-xCoO3-y compounds were prepared by a cold high-pressure procedure, and their thermoelectric (TE) properties were investigated. Compared with the ion-substituted hole-type LaCoO3 systems (e.g., La1-xSrxCoO3), the thermal conduction of La1-xCoO3-y is noticeably reduced by the La/O vacancies, whereas the electric transport is less influenced, which results in an efficient ZT enhancement. We demonstrate that the large thermal conductivity reduction originates from the strong point-defect scattering, and La1-xCoO3-y can be rationalized as a partially filled solid solution: La1-x square xCoO3-y square(y), where square denotes a vacancy. Such intrinsic thermal conductivity suppression provides an effective pathway for the design of better TE materials.