The bandgap engineering of graphene is a challenging task for its potential application. Forming unique structures such as nanoribbons or nanomeshes is an effective way to open up a bandgap in graphene. In this work, a graphene nanomesh (GNM) was prepared through UV-mediated oxidation of a graphene oxide (GO) film at atmosphere. Atomic force microscopy (AFM) was used to track the evolution of the surface morphology of GO during the irradiation. It was observed that a nanoporous network structure was progressively produced in the basal plane, which can be attributed to the fact that highly reactive oxygen species preferentially attack sp3 carbon-rich regions of the GO. In particular, the as-prepared GNM shows interesting semiconducting characteristics and photoluminescence (PL) phenomenon, which make it become a promising candidate for the use of electronics, optoelectronics, and biomedical engineering. Finally, the field-effect transistors (FETs) were fabricated using the as-prepared GNM as the active channel. The measured electrical characteristics indicate that the use of UV/O3 is an available choice to open the bandgap of graphene and tune its properties for optoelectronics or biomedical applications.