In order to improve the interface strength in the bonded body of hydroxyapatite (HA) ceramics and Ti disks prepared by a hydrothermal hotpressing (HHP) method, the effects of Ti surface modification on the bonding behavior were investigated. The reaction layer composed of titanium dioxide and sodium titanate was formed on the Ti surface using a 5 M NaOH solution with the objective of increasing the interface strength between the Ti substrate and HA ceramics to be formed by the HHP method. Three conditions with different temperature and treatment times were tested to modify the Ti surface. A mixture of calcium hydrogen phosphate dihydrate and calcium hydroxide was used as a starting powder material for solidifying HA. Solidification of HA and its bonding with Ti were achieved simultaneously by using the HHP method at the low temperature as low as 323 K. 3-point bending tests were conducted to obtain an estimate of the interface fracture toughness of HA/Ti. The Ti surface modification conducted at 323 K for 2 h using the hydrothermal NaOH solution was shown to be most effective among the three conditions tested. The hydrothermal Ti surface modification enabled us to increase significantly the interface fracture toughness. The enhancement of the interface fracture toughness was possibly due to the presence of anatase formed on the Ti surface and the good adhesion in the bioactive layer.