Potassium titanyl phosphate KTiOPO 4 (KTP) single crystal were implanted at room temperature with 300 keV Er +-ions in the dose range from 6 × 10 12 to 3.4 × 10 15 ions/cm 2. Annealing of the samples was performed in a furnace and also by means of a graphite strip heater. Rutherford backscattering experiments detect amorphization in the implanted layer for fluences ⪸ 1 × 10 13 Er + cm −2. Residual disorder detected after annealing of amorphous layers indicates the formation of microtwin structures in the as-implanted layers. Erbium is incorporated in the KTP-lattice with concentrations around 6 × 10 19 atoms/cm 3 if annealing is performed at 850°C.