Abstract In this paper, the fluid flow pass two heated/cooled square cylinders in a tandem arrangement is simulated via the Multiple-Relaxation-Time lattice Boltzmann method. The distance between the upstream and downstream cylinder varies from the rear of the upstream one to 5 times of the cylinder width. The numerical experiments are done under different Richardson numbers (Ri, represents the effect of the buoyancy force) for two typical Re=100, 60. The buoyancy effect on the flow and heat transfer around the two cylinders is mainly investigated. As is shown, if the force is in the same direction of incoming flow, the vortex street is always suppressed and no critical spacing seems to exist. However, if the force is in the opposite direction of the incoming flow, the vortex street can always be generated and the critical spacing always seems to exist. Correspondingly, the heat transfer around the cylinders measured by the Nusselt number on the surfaces of the two cylinders also shows different characteristics for various Ri s.