The effect of FIV on forced convection heat transfer from two elastically mounted tandem cylinders of unequal diameters has been investigated. The present numerical simulation demonstrates the effect of the diameter ratio (d/D = 0.2–1) on the heat transfer of an upstream cylinder with variable diameter ‘d’ and a downstream cylinder with constant diameter ‘D’. The flow is considered incompressible, two-dimensional, laminar at Re = 150 and Pr = 0.71. The two elastically mounted heated cylinders are allowed to oscillate in the transverse direction with reduced velocity Ur = 4 and 6, mass ratio m* = 2, damping ratio ξ = 0. The gap ratio between cylinders G/D = 1.5 and 3.5 are considered to elucidate the effect of heat transfer in extended body regime (G/D = 1.5) and reattachment regime (G/D = 3.5). The impact of the diameter ratio (d/D = 0.2–1) on flow dynamics and heat transfer characteristics is demonstrated through oscillation amplitude, flow structures and Nusselt number.