Superparamagnetic nanofluids based on magnetic composite nanoparticles were obtained, and natural convection experiments were conducted to enhance heat transfer performance by controlling the mass ratio of materials and the magnetic field. A 2:1 mass ratio for Fe3O4 and CNT was selected to conduct natural convection experiments as it provides better cost and efficiency of heat transfer compared to those for mass ratios of 1:1 and 4:1. The heat transfer and flow of natural convection were controlled by the heating power and the strength of magnetic field. The efficiency of natural convection with Fe3O4@CNT nanofluids increased from 14.1% to 29.1%, when increasing the heating power of base fluids from 3.528 W to 14.364 W without magnetic field, it further increased from 6.7% to 18.4% with the external magnetic field increased from 5 to 40 mT. The influence of magnetic field on heat transfer was also studied by simulation of non-isothermal flow, which was consistent with the experimental results. This research provided an approach to control the convective heat transfer coefficients by changing the mass ratio of materials, heating power, and external magnetic field. (C) 2019 Elsevier Ltd. All rights reserved.