TiO2-based photocatalysis has been widely used to decompose various organic pollutants for the purpose of environmental protection. Such a "green" photochemical process can ultimately degrade organic compounds into CO2 and H2O under ambient conditions. We demonstrate here its extended application on the engineering of single- or few-layer graphene. Using a patterned TiO2 photomask, we have achieved various photochemical tailorings of graphene, including ribbon cutting, arbitrary patterning on any substrate, layer-by-layer thinning, and localized graphene to graphene oxide conversion. UV-visible spectroscopic studies indicate that the photogenerated, highly reactive center dot OH radicals work as sharp chemical scissors. Being a solution-free, cost-effective, scalable, and easy handling technique, the presented photocatalytic patterning and modification approach allows for the versatile design and fabrication of graphene-based devices and circuits, compatible with current microelectronic technology, as demonstrated by this fabricated all-carbon field effect transistor (FET) array.