In the present study, the effect of nanofluids on the thermal performance of double pipe counter flow heat exchanger equipped with a decaying swirl flow system is numerically investigated. The swirl flow is generated using a tangential slot at the flow inlet of the outer pipe with a tangent angle of 30º. The volume concentrations of four different types of nanoparticles, Al2O3, CuO, TiO2 and ZnO in the range of 0% to 3% and different nanoparticle diameters in the range of 20 nm to 50 nm are considered for each in this study. The results indicate that, the heat transfer coefficient and pressure drop increase with increasing Reynolds number and volume concentrations of nanofluids. All nanofluid types achieve better heat transfer enhancement compared to pure water with slight increases in the pressure drop. The Al2O3 nanomaterial has better thermal enhancement characteristics followed by CuO, ZnO, and TiO2, respectively. The average heat transfer coefficient enhancement is 14% for the Al2O3 nanoparticles with a volume concentration of ϕ=3% and nanoparticle diameter of 32 nm, while the increase in pressure drop reaches 5% and 72% of pure water for the Al2O3 nanoparticles with a volume concentration of ϕ=0.5% and ϕ=3%, respectively. As the particle diameter decreases, the heat transfer as well as the effectiveness increase and there is a slight variation in the pressure drop. Finally, the use of nanofluids can be suggested as being effective method for enhancing the performance of heat exchanger