This paper presents a numerical study of nanofluids condensation heat transfer inside a single horizontal smooth square tube. The numerical results are compared with the previous experimental predictions. The numerical results show that the heat transfer coefficient could be improved within 20% by increasing the volume fraction of Cu nanoparticle by 5% or by increasing the mass flux from 80 to 110 kg/m2 s. Reducing the hydraulic diameter of the microchannel from 200 to 160 µm leads to an increase in the condensation average heat transfer coefficient by 10%. A new correlation estimating the Nusselt number for the condensation of nanofluids or pure vapor is proposed. It predicts average condensation heat transfer with a good agreement with those computed.