Coal tar pitch (CTP), a by-product of coking industry, has a unique molecule structure comprising an aromatic nucleus and several side chains bonding on this graphene-like nucleus, which is very similar to the structure of graphene quantum dots (GQDs). Based on this perception, we develop a facile approach to convert CTP to GQDs only by oxidation with hydrogen peroxide under mild conditions. One to three graphene layers, monodisperse GQDs with a narrow size distribution of 1.7 ± 0.4 nm, are obtained at high yield (more than 80 wt. %) from CTP. The as-produced GQDs are highly soluble and strongly fluorescent in aqueous solution. This simple strategy provides a feasible route towards the commercial synthesis of GQDs for its cheap material source, green reagent, mild condition, and high yield.