[KNbO3](0.9)[BaNi1/2Nb1/2O3](0.1) (KBNNO) perovskite ferroelectric thin films with narrow band gap (up to 139 eV) and good ferroelectric properties are prepared successfully by pulsed laser deposition. The emergence of Ni 3d gap states is confirmed to be responsible for the narrowing down of band gap of KBNNO, which correlates almost linearly with the increase of Ni content. Via a linear fit, a quantitative relationship between the Ni content and the band gap is established, laying a foundation for future design of new KBNNO-based materials with narrow band gap. The room-temperature remnant polarization (up to 1.4 mu C/cm(2)) of the KBNNO thin film is by an order of magnitude higher than that (0.1 mu C/cm(2)) of previously reported thick film with 20 mu m thickness. Both narrow band gap and large room-temperature remnant polarization make KBNNO thin films very attractive for developing ferroelectric photovoltaic devices with high conversion efficiencies. (C) 2017 Elsevier Ltd. Ali rights reserved.