Polarization rotation induced by an external electric field in piezoelectric materials such as PbZr1-xTixO3 is generally regarded as the origin of their large piezoelectric responses. Here, the piezo electric responses of high-quality cobalt-substituted BiFeO3 epitaxial thin films with monoclinic distortions are systematically examined. It is demonstrated that polarization rotation plays a crucial role in improving the piezoelectric responses in this material.