This paper presents the results of a numerical study on unsteady mixed convection flow of nanofluids in lid-driven enclosures filled with aluminum oxide and copper-water based nanofluids. The governing equations are solved by the Dual Reciprocity Boundary Element Method (DRBEM), and the time derivatives are discretized using the implicit central difference scheme. All the convective terms and the vorticity boundary conditions are evaluated in terms of the DRBEM coordinate matrix. Linear boundary elements and quadratic radial basis functions are used for the discretization of the boundary and approximation of inhomogeneity, respectively. Solutions are obtained for several values of volume fraction (phi), the Richardson number (Ri), heat source length (B), and the Reynolds number (Re). It is disclosed that the average Nusselt number increases with the increase in volume fraction, and decreases with an increase in both the Richardson number and heat source length. (C) 2013 Elsevier B.V. All rights reserved.