A 500 A˚ erbium (Er) film was deposited on potassium titanyl phosphate (KTiOPO4or KTP) by electron‐beam evaporation. The doping of Er into KTP was induced by mega‐electron‐volt Ar+with different doses. The doping amount was investigated by Rutherford backscattering of 3.0 MeV He+. The results show that the doping concentrations of Er in KTP were 2.88 and 1.88 at. % induced by 2.0 and 3.0 MeV Ar+with the dose of 2×1016ions/cm2at room temperature, respectively. Perhaps the present result can provide an important approach not only for the laser and waveguide laser fabrication, but also for the high‐concentration dopants (atomic percent levels) into KTiOPO4at room temperature. ©1995 American Institute of Physics.