H+-implantution in Bi4Ge3O12 (BGO) crystals causes radiation damage, optical absorption and decomposition of the near-surface region of the crystals. The colour of the BGO samples after H+-implantation changed to brown, but if is failed to prove that this change was caused by the producing of colour centers. In addition, no ion beam-induced optiCal activity was experimentally observed in H+-implanted BGO. It implied that there was no structure phase transition from Bi4Ge3O12 to Bi12Ge O20 version during H+-implantation. It can be assumed that ion beam-induced decomposition may happen during ion implantation. Optical waveguide has been formed in BGO substrate by H+-implantation. There was a refractive index enhancement in the implanted BGO sample, specially for blue light (0.488 μm). It might be resulted from the ion beam-induced decomposition.