Helium-ion-induced radiation damage in a LiNbO3-thin-film (10 μm-thick) modulator is experimentally investigated. The results demonstrate a degradation of the device performance in the presence of He(+) irradiation at doses of ≥ 10(16) cm(-2). The experiments also show that the presence of the He(+) stopping region, which determines the degree of overlap between the ion-damaged region and the guided optical mode, plays a major role in determining the degree of degradation in modulation performance. Our measurements showed that the higher overlap can lead to an additional ~5.5 dB propagation loss. The irradiation-induced change of crystal-film anisotropy(n(o)-n(e))of ~36% was observed for the highest dose used in the experiments. The relevant device extinction ratio, V(π)L, and device insertion loss, as well the damage mechanisms of each of these parameters are also reported and discussed.