The effect of adding carbon nanotubes (CNT) into epoxy on the temperature gradient in thick thermoset composites was studied and presented. Addition of CNT increases the thermal diffusivity of the resin and reduces the curing reaction speed. The latter slows down the rate of energy liberation, while the former helps to dissipate faster the released heat in the exothermic reaction. The results showed that the addition of up to 1 wt% CNT can reduce the difference between temperatures at the center and at the surface of 1.5-inch thick column of epoxy by 41%. Measured variations of heat capacity and thermal diffusivity by changes in both temperature and carbon nanotube contents as well as the empirically-evaluated cure kinetics of epoxy were used in a transient one-dimensional heat transfer finite difference model to determine the temperature distribution across thickness during the cure. Good agreement was obtained between calculated and experimental trends.