Graphene quantum dots (GQDs) as a new series of nanomaterials have drawn great attention in recent years owning to their unique properties. Here we report the single-step synthesis of GQDs using pyrolysis of citric acid which produced GQDs at different pH. The effect of different pH was studied in detail to optimize the conditions of the formation of GQDs. UV-Visible absorption and normalized fluorescence spectra were applied to analyze the optical properties of GQDs. The particle size distribution of the GQDs in case of varying pH was determined to optimize the synthesis conditions. The surface morphology and microstructures were studied by atomic force microscope (AFM).