An on board system for wave data analysis and application to ship operation has been developed. The system consists of wave probes, accelerometers, a soft-ware of analysis and a wave data display tool. The former two are used to estimate the encountering wave surface elevations on ship hull. The main parameters of waves, i.e. the wave direction, the mean wave period or wave length and the significant wave height are calculated by using the encounter wave elevation data at three positions on ship side. The obtained wave parameters are used to show the situation of own ship at sea on the figures of operational guidance, so that the master can judge how to change the ship course or speed for preventing dangerous situations. This paper presents a simple method of wave analysis using cross spectra of wave data at three positions on ship sides and the examination of the method by numerical simulations and model experiments in directional waves. The test results on actual vessel at sea will be presented.