In this work, mixed convection of Cu-water nanofluid in a trapezoidal enclosure with heat source on lateral walls has been numerically investigated. Vertical walls of the enclosure are kept at constant temperatures of Th and Tc, while horizontal walls are insulated. The mixed convection flow has been generated by passing the fluid through the enclosure and natural convection has been, also, investigated by holding the left wall at a temperature higher than the right wall. In order to examine the effect of the ports position, two cases were considered. Comparison between the results indicates that the rate of heat transfer is higher when the inlet port is near the cold wall than the hot wall. The results have been presented for various volume fractions, Richardson and Reynolds numbers. It was observed that for the considered Reynolds numbers and Richardson number, at a given Reynolds number and solid volume fraction, the Nusselt number increases with increasing the Richardson number. Moreover, at a given Richardson number and solid volume fraction, increasing the Reynolds number results in an increase in the Nusselt number. For the higher Richardson and Reynolds numbers, the nanofluid has more effect on the increase of the heat transfer performance.