The present study investigates the heat transfer enhancement mechanism observed with a non-Newtonian fluid having temperature-dependent viscosity in a 2:1 rectangular duct. The Reiner-Rivlin constitutive equation was used to model the non-Newtonian fluid characteristics. The H2 thermal boundary condition, corresponding to an axially-constant heat flux with a uniform heat flux at the top wall, was used. The local Nusselt numbers calculated for a polyacrylamide (Separan AP-273) solution showed significant heat transfer enhancements over those of a constant property fluid and gave excellent agreement with experimental results in both the regions of thermally developing and fully developed. The heat transfer enhancement results from an increased fluid mixing near the heated top wall, which is attributed to both the effects of the temperature-dependent viscosity and secondary flow induced by second normal stress difference. The present study concludes that the heat transfer enhancement of the viscoelastic fluid in a 2:1 rectangular duct is caused by the favorably combined effect of temperature-dependent viscosity and normal stress-induced secondary flow.