Experimental studies on the laminar heat transfer of 4000 wppm and 8000 wppm aqueous carboxymethylcellulose(CMC) solutions in a top-wall heated square duct have been carried out. The square duct is 240 cm long and its hydraulic diameter is 0.4 cm. Results show that the aqueous CMC solutions are good candidates for laminar heat transfer enhancement in a non-circular duct. Under the examined conditions, the enhanced local heat transfer along the whole duct wall can be characterized by a two-zone model with a Newtonian heat transfer section followed by a viscoelastic heat transfer section. The fully developed heat transfer of the aqueous CMC solutions can be well determined by linear correlations with the Kozicki generalized Reynolds number.