In this study, the effect of constant magnetic field on the laminar force convective heat transfer of a hybrid nanofluid containing Fe 3 O 4 nanoparticles and carbon nanotubes (CNTs) flowing through a heated annular tube were investigated numerically. The numerical investigations were carried out over a wide range of parameters such as volume concentration of Fe 3 O 4 (0.5-0.9%) and carbon nanotube (0.28-0.9%) nanoparticles, Reynolds number (548-1643), magnetic field strength (300-1000 Gauss), heat flux ratio (0-1), movement of iron belts along horizontal annular tube and radius ratio (0.4-0.6) on the laminar force convective heat transfer of aforementioned ferrofluid. To perform a numerical investigation, first, the magnetic field is simulated using Maxwell software, and then the results are transferred to a numerical code written in FORTRAN. In the present study, the numerical simulation observation in the case without magnetic field revealed that by using Fe 3 O 4 /CNT hybrid nanofluid, the convective heat transfer has been improved significantly. The maximum enhancement of 23.1% and 4.03% were achieved in the local Nusselt number at the outer and inner walls respectively for hybrid nanofluid containing 0.9 vol.% Fe 3 O 4 and 1.35 vol.% CNT at Reynold number of 1643. Additionally, the results showed that the heat transfer of the studied hybrid nanofluids has been improved in the presence of constant magnetic fields. Moreover, the effects of magnetic field were more noticeable in the hybrid nanofluids with higher volume concentrations and lower Reynolds number. For example increase in parameters such as magnetic field strength, higher volume concentrations of Fe 3 O 4 and CNT nanoparticle and heat flux ratio and a decrease of parameters such as radius ratio and Reynolds number resulted in heat transfer augmentation. Eventually, the highest enhancement of 34.35% and 2.95% in comparison with the case without magnetic field were reported in the local Nusselt number at the outer and inner walls respectively for hybrid nanofluid containing 0.9 vol.% Fe 3 O 4 and 0.9 vol.% CNT at Reynolds number of 548 and magnetic field strength equals to 1000 Gauss. Keywords: Ferrofluid, Carbon Nanotube, Magnetic Nanoparticles, Constant Magnetic Field, Convective Heat Transfer.