The high-quality bulk potassium titanyl phosphate (KTiOPO4 or KTP) crystals have been grown successfully by the hydrothermal method using an improved mixed solution of K2HPO4 (2.0 mol/1), KH2PO4 (0.1 mol/l and H2O2 (1 wt%) as mineralizer, and spontaneous flux KTP crystals as nutrients. The growing rate of KTP crystals along (011) was 0.15 mm/day. The size of the biggest grown crystal was up to 26 x 83 x 25 mm 3, 132 g in weight. The transmittance was over 80% in the range of 450-2500 nm. The optical homogeneity was up to 4.15 x 10(-6) cm(-1). The precise principal refractive indices were measured by the auto-collimation method and compared with those of KTP grown by the flux method. The Sellmeir equations were fitted with the experimental data. The test of bulk absorption at strong CW green radiation 532 nm was performed with the photo-thermal common-path interferometer. The results showed that the hydrothermal KTP crystals have great ability to anti-gray track. (c) 2008 Elsevier B.V. All rights reserved.