In this paper, natural convection flow and heat transfer of Cu-water nanofluids in a horizontal triangularcylindrical annulus is investigated numerically. The inner circular cylinder is maintained at a constant high temperature of T-H whereas the outer triangular cylinder is kept at a constant low temperature of T-c. The governing equations are derived in the form of stream function-vorticity formulation and solved using control volume based finite element method (CVFEM). Theoretical models of Maxwell-Gametts (MG) and Brinkman are employed to predict the thermal conductivity and viscosity of the nanofluid respectively. The effects of Rayleigh number, nanoparticles and annulus radius ratio on fluid flow and temperature field are plotted and discussed. The results indicate that the average Nusselt number increases with increasing the radius ratio at higher Rayleigh numbers, whereas it decreases at lower Rayleigh numbers. It also reveals that heat transfer is augmented by adding the nanoparticles to the base fluid. Moreover percentage of heat transfer enhancement caused by the nanofluid is more remarkable at lower Rayleigh numbers. (C) 2015 Elsevier B.V. All rights reserved.