A new type of tether cable with high quality signal transmission characteristics was developed by adopting optical fibers. As the tether cable is subject to frequent and various mechanical and thermal loads under the severe operating conditions in the offshore environment, the greatest care should be paid to the design of its structure to preserve the optical fibers from those loads. This paper describes the design requirements and the characteristics of the optical fiber/power line composite tether cable under the loads such as tensile loading, bending, twisting, water pressure, and cable heating. The termination of the tether cable is also discussed. This tether cable has been in practical use for remotely operated submersible MARCAS whose mission is inspection and location of the submarine telecommunication cables.