An experimental and CFD investigation is performed to evaluate laminar forced convective heat transfer and friction factor at various concentrations of Fe3O4 nanofluid (NF) in a plain tube with twisted tape (TT) inserts under different uniform electromagnetic flux density. In this paper, a novel compound heat transfer augmentation techniques are investigated. A very few studies are found in the literature with this combination of techniques. So, it is necessary to investigate the flow physics of nanofluid with twisted tape inserts under the effect of magnetic field. The twisted tapes used in the experiments are classical twisted tape having a twist ratio of 3.7. Investigations are carried out with the NF powder volume concentration of 0.05%, 0.1%, 0.15% and 0.2% of Fe3O4. The Reynolds number is varied from 900 to 1800 under constant heat flux of 2300 W/m2. The uniform electromagnetic field of 10, 20 and 30 Gauss is applied to the test section. It is observed that the performance of twisted tape with magnetic field and Fe3O4 NF is higher than twisted tape with distilled water at all the Re numbers. The heat transfer enhancement in terms of the Nusselt number values for 0.05%, 0.1%, 0.15% and 0.2% of Fe3O4 NF at Re of 1800 along with TT inserts and magnetic field of 30 Gauss increased by 209%, 219%, 226% and 232%, respectively, over that of distilled water in plain tube. It can be seen that the heat transfer coefficient increases with an increase in percentage volume concentration of NF and magnetic field intensity for all the cases. But this causes the increase in friction factor due to addition of the NF and magnetic field intensity. So, the thermal performance factor (TPF) is evaluated to analyse the overall performance of heat transfer enhancement for each working condition. In case of 0.20% volume concentration of NF at Re = 1800 and magnetic field intensity of 30 G with TT inserts, the highest value of TPF observed is 2.56. From the experimental results, it is observed that for all the cases investigated, a high nanoparticle percentage volume concentration, high magnetic flux density and TT inserts provide higher thermo-hydraulic performance. It can be used in various applications of heat exchangers like automobile radiator cooling, data centre cooling, nuclear reactor cooling, etc. © 2021, King Abdulaziz City for Science and Technology.