A technique for motion estimation of a tether cable of an observation class ROV is proposed in this paper. A simulation procedure based on the lumped mass method is adopted for the motion and shape estimation of the cable. Deployment of the tether cable from a starting point is approximately treated locating the mass points at the starting point. These mass points are deployed and follow the leading part of the cable. The simulation results are compared with results from tank experiments and the validity of the simulation technique is confirmed. Examples of path planning for the inspection of a pontoon type floating structure is also shown.