Lattice distortions under an electric field in a mono-domain of BaTiO(3) ferroelectric crystal have been detected with synchrotron x-ray radiation. The variation of the lattice constant with an electric field observed with high angle diffraction shows a linear response nature of the piezoelectric effect. When an electric field is applied along the spontaneous polarization direction, the c-axis of the lattice elongates and the a-axis of the lattice shrinks at a rate of d(33) = 149 ± 54 pm V(-1) and d(31) = -82 ± 61 pm V(-1); these represent the longitudinal and transverse piezoelectric coefficients of BaTiO(3) crystal, respectively. These results give an insight into the intrinsic piezoelectric response on the lattice scale in BaTiO(3) that has been widely used to explore high performance lead-free piezoelectric alloys.