The　principal　refractive　indices　and　thermal　refractive　index　coefficients　for　Nb:KTiOPO4　(KTP)　crystal　grown　by　the　top-seed　flux　method　from　a　starting　material　containing　7.5　mol　%　Nb　have　been　measured　using　the　autocollimation　method.　Using　measurements　made　at　wavelengths　of　0.53975,　0.6328,　1.0795,　and　1.3414　mu　m,　in　the　temperature　range　from　ambient　to　188　degrees　C,　it　is　possible　to　determine　the　constants　of　the　Sellmeier　equations　and　the　thermal　refractive　index　coefficients　as　a　function　of　wavelength.　Also,　the　temperature　acceptance　bandwidth　at　90　degrees　second　harmonic　generation　noncritical　phase　matching　direction　was　calculated.　Comparison　of　these　results　with　those　for　undoped　KTiOPO4　[K.　Rato,　IEEE　J.　Quantum　Electron　QE-28,　1974　(1992)]　and　KNbO3　[B.　Zysset,　I.　Biaggo,　and　P.　Gunter,　J.　Opt.　Soc.　Am.　B　9,　380　(1992)]　implies　that　Nb-doped　KTP　crystals　are　suitable　for　blue　coherent　light　devices.　(C)　1999　American　Institute　of　Physics.　[S0021-　8979(99)08019-6].