Potassium titanyl phosphate (KTP) is mostly used in all-solid-state lasers as a good nonlinear optical crystal. Knowledge of variations in the weak absorption of different types of KTP crystals is important for the application. In this study, the flux-grown KTP crystals with titanium from TiO2, TiCl4, and Ti(OC4H9)4, respectively, and the hydrothermal-grown KTP crystal were investigated. The weak absorption values of the KTP crystals were measured at 1064 nm and 532 nm by photothermal common-path interferometer. The weak absorption values of four samples were 100 ppm·cm−1, 800 ppm·cm−1, 45 ppm·cm−1, 1064 ppm·cm−1 at 1064 nm, and 2.6×104 ppm·cm−1, 1.6×105 ppm·cm−1, 1.2×104 ppm·cm−1, 2.25×102 ppm·cm−1 at 532 nm, respectively. The results showed that the weak absorption values of the flux-grown KTP with titanium from Ti(OC4H9)4 and the hydrothermal-grown KTP were low values at 1064 nm. In addition, the weak absorption value of the hydrothermal-grown KTP was at least two orders of magnitude lower than that of the flux-grown KTP at 532 nm. From the results, we concluded that the KTP grown by titanium from Ti(OC4H9)4 represented a better quality than the other two flux-grown KTP crystals. The quality of the hydrothermal-grown KTP was obviously better than the flux-grown KTP.