This paper investigates the buoyancy driven mixed convective flow and heat transfer characteristics of water-based nanofluid past a circular cylinder in cross flow using a SUPG based finite element method. Nano sized copper particles suspended in water is used with Prandtl number (Pr) = 6.2, and the range of solid volume fractions 0 <= phi <= 25% are considered. Computations are carried out for the range of Reynolds number 80 <= Re <= 180. Effect of aiding and opposing buoyancy is bought about by considering two representative Richardson numbers of 1 and -1. Increase in nanoparticle loading show symmetric vortex structure distributions and have minimal effect of negative buoyancy. Width of the flayers of thermal energy reduces with increasing nanoparticle volume fractions. Increase in solid volume fraction show reduction in thermal boundary layer thickness and increased thermal gradient at the cylinder surface. The local and average Nusselt numbers are found to increase with increasing Re and phi. The average Nusselt number at Ri = 1 and Ri = -1 are found to vary as 0.947Re(0.550)(1 - phi)(-1.6253) and 0.881Re(0.559)(1 - phi)(-1.7349) respectively. It is observed that the presence of nanoparticles imparts a counter balancing force to that of buoyancy force. This tries to minimize the effect of buoyancy force and stabilizes the flow. This is, perhaps, the first time that such behavior for the nanofluid is being reported. (C) 2012 Published by Elsevier Ltd.