Optical third harmonic radiation has been measured in reflection from Surfaces of Single crystal and ion implanted silicon samples. As was proved in our theoretical Calculation, the linearly polarized third harmonic intensity exhibits the rotational symmetry of the crystal as the crystal rotates about its surface normal. There is a critical implant etose in lightly implanted samples. The rotational dependence of the thirel Larmonic radiation is found to vanish when the ion dose exceeds the critical value. As a function of the implant dose, the third harmonic intensity is very sensitive to lattice damage. It is possihle to use the third harmonic technique for monitor of ion implant uniformity in semiconductor device technology.