This study deals with the numerical solution of steady laminar mixed convection flow in a lid-driven square cavity with a triangular heat source filled with water-based nanofluid. The left and down walls of the enclosure are kept insulated, the top wall moving at constant speed is maintained at constant temperature and the right wall is kept at constant temperature. The governing equations are presented in terms of the stream function-vorticity formulation and are solved numerically with suitable boundary conditions by a second-order accurate finite-difference method. A comparison with other works is done and a good agreement is obtained. A parametric study is done and graphical results are shown and discussed to show the effects of the volume fraction, nanoparticle diameter and different nanoparticle types on the flow and Nusselt number. It is found that suspending the nanoparticles in pure fluid leads to a significant heat transfer increase. (C) 2013 Elsevier B.V. All rights reserved.