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The thermal performance of the internal heat transfer fluid was an important factor affecting the heat transfer efficiency of the exchanger system. This study reports a nanofluid that achieves long-term stability without any additives. The synthesized environmentally friendly N-doped carbon quantum dots nanoparticles (N-CQDs) had a uniform particle size distribution, with particle sizes concentrated between 2 and 2.5 nm. The stability, thermophysical properties and rheological characteristics of N-CQDs nanofluids were investigated. The results showed that the Turbiscan stability index of N-CQDs nanofluid was about 1.5 and the ΔT value fluctuated within 0.5 %, showing excellent stability. The thermal conductivity of 0.1 vol% of N-CQDs nanofluid increased by 43.21 % at 50 °C compared to the base fluid. The specific heat capacity decreases with increasing nanofluid loading. The viscosity of N-CQDs nanofluid behaves as a Newtonian fluid. In addition, the models for estimating the viscosity and thermal conductivity of N-CQDs nanofluids are proposed, respectively. Due to the different particle sizes and shapes of nanoparticles, the proposed model can better predict the thermal conductivity and viscosity of N-CQDs nanofluids compared with other empirical models. [Display omitted] • The prepared N-CQDs nanofluids have excellent long-term stability. • The thermal properties of N-CQDs nanofluids have been enhanced. • The viscosity of N-CQDs nanofluids with different concentrations was measured. • The new models are proposed for estimating the thermal conductivity and viscosity of N-CQDs nanofluids. [ABSTRACT FROM AUTHOR]