The effects of thermal properties of Newtonian liquids on die swell were studied numerically using the finite element method. In particular, Newtonian liquids with temperature‐dependent viscosity were studied under variable operating conditions. The viscosity was assumed to be a decreasing exponential of the temperature, with a proportionality constant β which was assumed to be a given material property. Temperature and pressure distribution in the liquid, as well as the swelling ratio (extrudate diameter/capillary diameter) were found as a function of the constant β, Peclet number (Pe), and Nusselt (Nu) number. Two‐dimensional and axisymmetrical geometries were examined. The results show that a change in value of either β,Pe, orNutends to alter the intensity of the stress singularity at the lip of the capillary as indicated by the swell. An increase in β has very little effect on the temperature distribution, but tends to decrease the amount of swell. An increase in Peclet results in a hotter jet while increasing Nusselt results in a colder extrudate; in both cases, however, the amount of swell increases.