Single-walled carbon nanotubes (SWNTs) decorated with zinc oxide (ZnO) nanoparticles show remarkable UV photoresponse and are applicable as UV photodetectors. It is generally accepted that the UV response originates from the adsorption–photodesorption of oxygen from the surface of ZnO nanoparticles. We investigated the UV photoresponse properties of ZnO–decorated SWNTs, which have ZnO layers of various thicknesses and surface morphologies prepared by controlling the pulsed laser deposition (PLD) duration. The magnitude of the negative photocurrent and the recovery time were strongly dependent on the thickness and morphology of the ZnO layer, and were highest when the ZnO layer was 3–4nm thick. We revealed that there are two types of adsorption sites for oxygen on the ZnO layer, which are assigned to the oxygen-vacancy defect site and flat surface site, and that their abundances vary with the thickness and morphology of the ZnO layer, which affect the UV photoresponse properties of ZnO–SWNT devices. The present result suggests that, by controlling the number of defect sites on the ZnO surface, the photoresponse of ZnO–SWNT devices can be improved.