An experimental investigation of heat transfer to highly viscous pseudoplastic fluids (K from 1. 6 to 13. 7 Pa·sn) in vertical tubes under constant heat flux was carried out. Experimental data of heat transfer through the flow and thermally developed region in 0. 016 and 0. 021m diameter pipes were collected and were compared with some previous work. The correlation taking into account the effect of both radial and axial variation of viscosity on heat transfer agreed with experimental data very well. Natural convection effect was insignificant because of high viscosity.