Nitrogen-doped graphene (NDG) nanofluids are prepared using a two-step method in an aqueous solution of 0.025 wt% Triton X-100 as a surfactant with various nanosheets at several concentrations (0.01, 0.02, 0.04, 0.06 wt%). The results are reported of experiments on the thermal conductivity, viscosity and convective heat transfer behavior of NDG nanofluids undergoing laminar flowing in a circular tube. The results indicate that, compared to the base liquid, the thermal conductivity is enhanced for NDG nanofluids by between 22.15% and 36.78%, and the heat transfer coefficient of the NDG nanofluids is increased by 7-50%. The measurements also show that the pressure drop of the nanofluids increased by between 0.08% and 14.4%. In addition, the overall performance of the tested nanofluids are assessed based on the performance index and optimum work conditions, demonstrating that the nanofluids can be advantageous in practical applications. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.