Heat transfer rate plays a very important role in improving the performance of a vehicle radiator. Use of nanofluid as a coolant improves heat transfer rate as well as the performance of the automobile radiator. Numerical investigations have been carried out to study the effect of carboxyl graphene nanofluid in an automobile radiator. A horizontal flat tube radiator with rectangular fins has been modeled using SolidWorks 2017^®. A CFD analysis is carried out using ANSYS Workbench 19.2^® to predict the heat transfer rate and pressure drop in the radiator. Simulations were carried out using 50:50 water-ethylene glycol mixture, and carboxyl graphene nanofluids of 0.02%, 0.03%, and 0.04% volume concentrations with 50:50 water-ethylene glycol as base fluid at inlet temperature of 40 °C. The effect of different cooling fluids on performance parameters such as friction factor, Nusselt number, effectiveness and NTU were investigated and compared. It was observed that the nanofluid with highest nanoparticle concentration (0.04%) had the highest Nusselt number, pressure drop, friction factor and effectiveness.