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　μm,　in　the　temperature　range　from　ambient　to　188°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°　second　harmonic　generation　noncritical　phase　matching　direction　was　calculated.　Comparison　of　these　results　with　those　for　undoped　KTiOPO4　[K.　Rato,　EEEE　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.　©　1999　American　Institute　of　Physics.