Abstarct: The subject of efficiency of heat exchangers has been taken into a specific concern by researchers. In the past in order to increase the heat transfer used to be techniques like adopting fins or creating vortex generator on the flow pass and change of the geometry of the exchanger. But today there needs to be an improvement to the heat transfer. Therefore researchers are trying new techniques to increase efficiency of heat exchangers. Using nanofluids instead of which is used in heat exchangers is one of the most widely used methods. In this experimental research we study heat transfer in force convection mode for iron oxcid as a nanoferrofluid in a U-shape copper pipe under the effect of magnetic field; the surface of the pipe is heated by an electrical heater and the constant heat-flux boundary condition is applied. Iron-oxcid nanofluid has been studied in 0.2% & 0.4% volume fraction, in Reynolds number limit 260-930. Also tests have been done in 2 different curve radiuses which allows us to compare the two modes with different Dean Number in same Re.No, also pressure drop in both modes. The results show a remarkable increase of heat transfer when 0.4% volume nanofluid under the effect of an alternating magnetic field is used, in this mode Nusselt number increased about 23% compared to the mode when water is used. But considering pressure drop for each pipes it was revealed that pressure drop was more when Reynolds number is the same and small curve radius pipe is used. Therefore after introducing a general efficiency for the system, it was observed that for the pipe with a larger curve radius and 0.4% volume nanofluid under the effect of alternating magnetic field which resulted in general efficiency of 1.12 that is considered as maximum efficiency in all modes. Finally after investigating all modes it was revealed that only in few modes the general efficiency of system is higher than 1; so for designing the heat exchangers in shape of curve pipes, applying magnetic fields will cause hydrodynamical problems to the system and leads to a high increase of pressure drop. But heat efficiency will increase by applying magnetic field, increase of nanofluid volume fraction & increase of Dean Number.