The paper reports the measurement of refractive indices and anisotropic absorption coefficients of biaxial potassium titanyl phosphate (KTP) crystal in the form of thin plate using reflection ellipsometry technique. This experiment is designed in the Graduate Optics Laboratory of the Addis Ababa University and He–Ne laser ($\lambda = 632.8$ nm), diode laser ($\lambda = 670.0$ nm) and temperature-tuned diode laser ($\lambda = 804.4$ and 808.4 nm), respectively have been employed as source. The experimental data for $n_{x}$, $n_{y}$ are fitted to the Marquardt–Levenberg theoretical model of curve fitting. The obtained experimental data of refractive indices are compared with different existing theoretical and experimental values of KTP crystals and found to be in good agreement with them.