Motions of a pilot boat in the vicinity of a large ship are investigated in order to design a stable pilot platform for minimizing the risk to pilots in the process of embarkation or disembarkation from 1arge ships in rough seas. A theoretical method is discussed, and theoretical values show a good agreement with results of experiments from model tests. With this method, the motions are estimated and it is found that 1) heaving amplitude is very large over a wide range of frequency. 2) in swaying motion there is not only collision to large ships but also drifting from them. 3) rolling amplitude shows a large value at the resonant frequency.