To measure the thermal conductivity of nanofluids under flow condition accurately, a novel method defined as the steady flow method (SFM) was employed and improved based on the heat transfer of laminar flow theory under the uniform heat flux condition. Considering the buoyance is existed in the actual pipe flow, Nusselt number would increase rather than a constant value of 4.364 in the fully developed region. The evaluation of Nu was optimized by experimental validation, and the mixed convection equation was used to improve Nu in the thermal conductivity measurement coupled with SFM. Then the thermal conductivities of water based nanofluids with 0.2 wt%, 0.5 wt% and 1 wt% Al2O3 were measured respectively by using the improved SFM (ISFM). Compared to the reference thermal conductivity obtained from REFPROP 9.1, the experimental results show that the maximum relative deviation on measured values of deionized water is reduced from 10.42% to 3.35% by ISFM. The measured thermal conductivities of water based nanofluids with 0.2 wt%, 0.5 wt% and 1 wt% Al2O3 are increased by 10.5%, 16.7%, 22.8% compared with the base fluid. Moreover, the thermal conductivity of nanofluids can be enhanced with the fluid temperature, which corresponds with other literatures. The ISFM reduces the influence of natural convection in the processes of measurement, and then leads to improving the measuring accuracy of the thermal conductivity significantly. (C) 2018 Elsevier Ltd. All rights reserved.