We have carried out the measurements of complex dielectric constants with impedance/gain-phase analyzer using capacitor method and the experiments of high temperature X-ray powder diffraction with X-ray diffractometer using ${ heta}-2{ heta}$ scan method for the KTP single crystal which has the premium nonlinear optical properties. From the results of high temperature X-ray powder diffraction experiments, we have found that KTP does not undergo structural phase transition below $900^{circ}C$. It is clear that KTP undergoes structural phase transition around $900^{circ}C$ and belongs to orthorhombic above $900^{circ}C$ still. However, we have applied phenomenological relation of dielectric relaxation to the results of complex dielectric measurement and have found that relaxation mechanism of KTP well satisfies the Cole-Cole relation over the temperature range from $-78^{circ}C$ to $200^{circ}C$. And also the relaxation time well satisfies the Vogel-Fulcher relation. It is regarded that the hopping and thermally activated diffusion mechanism may control the conduction behavior of KTP above $200^{circ}C$.