Effects of CuO on constrained sintering of a polycrystalline TiO2 ceramics have been investigated. The densification temperature of TiO2 is reduced from 1100-1200 degrees C for pure TiO2 to 900 degrees C with the presence of 0.5-3 mol% CuO under free sintering. For the samples with 1 mol% CuO, the constrained densification is slowed down, but a high sintered density of >95% at 950 degrees C, which is close to that sintered freely, is still obtained. The above results are caused by the formation of CuO-rich film at the grain boundaries, which reduces grain-boundary energy and enhances grain-boundary migration kinetics of TiO2. To confirm the above findings, molecular dynamics simulation, at which the ratio of grain boundary energy of TiO2 between with and without CuO agrees well with that obtained experimentally, is conducted.