The formation of hillocks was examined in various Pt/Ti electrodes with similar thermal histories, which were annealed prior to PZT film deposition by the sputtering method. The Pt electrode with no Ti underlayer did not exhibit hillocks. An increase of the Ti thickness or a decrease of the Pt thickness in the Pt/Ti electrodes were found to result in an increase in hillock density and the reduction of hillock size. A pre-annealing in Ar reduced the oxidation rate of Ti during the subsequent oxygen annealing, resulting in the reduction in density as well as the size of hillocks. The density and the size of hillocks were also reduced by a decrease of oxygen partial pressure during oxygen annealing. Hillocks in Pt/Ti electrodes are formed to relieve the compressive stress of the Pt films, which is mainly developed by the volume expansion of the Pt layer, due to the oxidation of diffused Ti along the Pt grain boundaries. A comparison of the short probabilities of the PZT films with the observed hillocks suggests that the electrical short of the PZT capacitors should not depend on the density, but on the size of hillocks.