Defects in graphene are of great interest as they are considered to provide various intriguing physical and chemical properties to graphene. We have investigated defect formation in graphene induced by UV-irradiation under various atmospheres. We have found that graphene can be damaged by UV-irradiation in vacuum. The defect formation by UV-irradiation was accelerated under ammonia ambience, while no additional effect was observed for UV-irradiation in nitrogen. The noticeable enhancement of defect formation could be attributed to a radical reaction where reactive species generated by photodissociation reaction of ammonia attacks the chemical bond of graphene. The UV-irradiation under ammonia ambience causes not only defect formation but also nitrogen doping to graphene. Incorporation of nitrogen atoms seems to occur rather locally to form graphitic carbon nitride domains. (C) 2013 Elsevier B. V. All rights reserved.