The present study deals with the investigation of the heat transfer performance of nanofluids using diesel oil as base fluid and multi-walled carbon nanotubes (MWCNT) and graphene nanoplatelets (GNP) as nanoparticles with different concentrations and flow rates. These nanoparticles have been functionalized covalently with hexylamine (HA) and non-covalently with oleic acid (OA) to prepare nanofluids to include OA-MWCNT, HA-MWCNT,OA-GNP.HA-GNP and OA-MWCNT/GNP (1-1 Hybrid of each of nanoparticles) at 5 weight percentages include 0.05-0.5 wt%. An experimental setup designed and constructed and heat transfer performance of the above-mentioned nanofluids are investigated under laminar flow conditions at 4 flow rates. In order to utilize nanofluids in practical applications, the main purpose of this study is focused on investigating if the addition of nanoparticles can enhance the heat transfer performance of diesel oil. In addition, the pressure drop and performance index of nanofluids measured in a closed loop. The results revealed that heat transfer properties of nanofluids improved compared to the base oil. Also according to the results, by increasing the velocity of nanofluid, the convective heat transfer coefficient increases at all weight coefficient of nanoparticles and an increase in pressure drop for nanofluids than pure oil is not perceptible. The performance index of nanofluids, except for the two cases that are related to the hybrid of MWCNT and GNP, in other cases is more than one. Therefore, using nanofluids leads to an increase in the heat transfer coefficient more than increasing the pressure drop, and are suitable for use in the industrial units. (C) 2019 Published by Elsevier B.V.