Increasing demands for LiNbO3 (LN) optical waveguide devices installed into submarine and terrestrial fiber communication systems necessitate that high quality and long-term reliability (over 20 years) be established. In addition to general requirements for reliability in electrooptical devices, the LN devices need to assure of the dc-drift phenomena in the optical output signal. The dc-drift is caused by the complex electrical nature of constituent device materials; i.e. LN substrates and SiO2 buffer layers on LN. Results of theoretical and experimental investigations of the suppression of dc-drift are applied to the design of practical LN devices, and the devices have been in service for 4 years, as of this moment, without any failure. Here, problems on the dc-drift are discussed from the viewpoints of device reliability and actual fabrication processes.