This study is concerned with mixed convection of water-based nanofluids in a lid-driven square enclosure with a constant heat flux heater. The governing equations are solved numerically using the differential quadrature method. The computational results are obtained for the heater lengths of 0.25, 0.50, and 0.75. The Grashof number is kept at a constant value of 10(4), and the Reynolds number is varied so that the Richardson number will have values in the range of 0.1 to 10. The nanoparticles volume fraction phi is varied as 0%, 5%, and 10% and the value of the ratio of the nanolayer thickness to the original particle radius eta is fixed to 0.1. The results show that the presence of nanoparticles in the base fluid causes a significant enhancement of heat transfer. The results also show that the heat transfer rate increases considerably with a decrease in the Richardson number and the length of the heater.