Being able to precisely control the reduction of two-dimensional graphene oxide films will open exciting opportunities for tailor-making the functionality of nanodevices with on-demand properties. Here we report meticulously controlled reduction of individual graphene oxide flakes ranging from single to seven layers through controlled laser irradiation. It is found that the reduction can be customized in such a precise way that the film thickness can be accurately thinned with sub-nanometer resolution, facilitated by extraordinary temperature gradients >102 K/nm across the interlayers of graphene oxide films. Such precisely controlled reduction provides important pathways towards precision nanotechnology with custom-designed electrical, thermal, optical and chemical properties. We demonstrate that this can be exploited to finely tune the work function of graphene oxide films with unprecedented precision of only a few milli electronvolts.