Effects of electron irradiation on the ferroelectric 180° in-plane nanostripe domain structure in a thin film prepared from a bulk single crystal of BaTiO3 by focused ion beam were studied. The domain structure transformed into a characteristic 90° in-plane nanostripe domain structure under intense electron irradiation. In particular, an unconventional triangular 90° in-plane nanostripe domain structure was observed. Polarization analysis suggests the existence of an incomplete or half tetradomain vortex at the boundary of the triangular domain structure. Together with the help of phase-field simulations using time-dependent Ginzburg–Landau equations, it is suggested that such a domain structure is created by an anisotropic in-plane electric field, which is plausibly induced by an anisotropic interaction of the incident electron beam with the ferroelectric material.