The features of the crystal structures and spontaneous polarization (Ps) under an electric field (E) have been reviewed for (1 − x)(Bi0.5Na0.5)TiO3–xBaTiO3 (BNT–BT). In-situ measurements of high-resolution synchrotron radiation X-ray diffraction (SR-XRD) under electric fields show that single crystals with x = 0 (BNT) and 5% have a monoclinic distortion in space group Cc at 25 °C. The SR-XRD study combined with density functional theory (DFT) calculations demonstrates that BNT–5%BT exhibits a rotation of Ps in the monoclinic a–c plane by 2° under an E of 70 kV/cm along the <001> pseudo-cubic direction, which is much larger than BNT.