In this study, new hybrid nanoparticles aggregates containing distinct iron-based and silicon nanophases are used to improve the thermal conductivity of the base fluid. The hybrid aggregates were synthesized using the pyrolysis laser technique and also characterized using XRD and TEM and HR-TEM analysis. The thermal conductivity of the hybrid nanofluid was measured at different temperatures within the range of 20-50 degrees C and mass concentrations of 025-1.0%. Based on the experimental results, a new correlation for the thermal conductivity variation with both the temperature and mass concentration of the Fe-Si hybrid nanofluids was developed. The experimental results were compared to experimental data from literature. Also, the correlations proposed by different researchers for thermal conductivity prediction of hybrid nanofluids were compared and analyzed. Results indicated that increased temperature and concentration lead to increases in thermal conductivity. (C) 2020 Elsevier B.V. All rights reserved.