In this study, we show photodynamic therapy (PDT) by graphene quantum dots (GQDs) and graphene oxide quantum dots (GOQDs) to kill the cancer cells. For PDT, we used GQDs and GOQDs as photosensitizer and 365 ​nm UV tubelight as irradiation source. GQDs/GOQDs are synthesized by electrochemical exfoliation of graphite rod. The electrolyte used was a combination of citric acid, NaOH and H2O2 in different proportion. The GQDs and GOQDs with the size range of 1.5–5.5 ​nm, show excitation dependent PL behavior. GQDs/GOQDs show strong absorbance in UV region, exhibit excitation dependent photoluminescence behavior ranging from UV to visible region and show singlet oxygen generation. GQDs/GOQDs show very low cytotoxicity towards skin cancer (B16F10) cells and breast cancer (MCF-7) cells. GQDs/GOQDs show easy cellular uptake and are well distributed inside the cells. Singlet oxygen generation, low cytotoxicity and easy cellular uptake make them a promising candidate for PDT application. PDT is performed on B16F10 ​cells and MCF-7 ​cells using GQDs/GOQDs. More than 90% cells are killed during a short exposure of 5 ​min. However, the cells not treated with GQDs/GOQDs and exposed to irradiation for 5 ​min show that more than 80% cells are alive. The GQD/GOQD PDT has several advantages such as high cell killing efficacy with micro-watt power UV irradiation, less irradiation time and uniform large area exposure. However, the therapy will be limited to skin related cancers as the depth of penetration of UV irradiation is very less. © 2019 Elsevier Inc.