We synthesized the hybrid structure of the graphene oxide (GO) sheet and the size-controlled ZnO nanoparticles. The ZnO quantum sizes were confirmed from UV-vis spectrophotometry and TEM results. The ZnO quantum dot exhibited a stable broad and strong photoluminescence (PL) emission which is broader with the increase of ZnO nanoparticle size. The hybrid ZnO/GO results an abrupt decrease of PL emission and the excellent photodegradation performance of methylene blue under UV-light irradiation. A considerable increase of the photodegradation rate with decreasing ZnO nanoparticle size is due to the enhancement of ZnO surface interaction and the improvement of electron transfer at the ZnO/GO interface.