We　report　a　novel　procedure　involving　polyethylenimine-assisted　hydrothermal　cutting　and　subsequent　ultrafiltration　for　fabricating　nearly　monodisperse　graphene　quantum　dots　with　a　uniform　lateral　size　and　confined　layer　number.　The　isolated　monolayer　quantum　dots　exhibit　a　sharp　band-edge　absorption　feature　and　strong　photoluminescence　(quantum　yield　of　21%)　independent　of　excitation　wavelength　and　pH.　Their　preliminary　application　in　bioimaging　has　been　demonstrated.