Graphene quantum dots (GQDs) have been prepared by hydrothermal cutting of graphene sheets and subsequently doped with N atoms by using hydrazine treatment to study the effect of doping on the structural and the optical properties of the GQDs. The pyridinic N atoms, which were increased in number by hydrazine treatment, are bonded with C atoms at the edge of the GQDs, thereby making the GQDs not only larger but also n-type conductive with a concomitant reduction of the edge-related defects. The absorption band of GQDs is peaked at ~300 nm, and after doping, they show an additional absorption peak at ~354 nm, which originates from the impurity energy level formed by the N doping. The GQDs emit bright blue photoluminescence (PL) peaked at 418 nm, and after hydrazine treatment, the PL emission is redshifted by ~15 nm and its decay becomes slower, resulting from the N-related impurity level.