Piezoelectric responses of barium titanate single crystals are investigated along the  and  directions, respectively, in order to verify the ultrahigh piezoelectric constant (2000 pC/N) reported in a previous study [Appl. Phys. Lett. 86, 012905 (2005)]. The results suggest that electric-field-induced strain can be largely enhanced by the domain switching in barium titanate single crystal along the  direction; however, the piezoelectric response under small field measurement has not been enhanced as much as expected because domain switching is a nonlinear and irreversible process.