The purpose of this study is to establish a method for evaluating ride quality of passenger ships. The authors have conducted an experiment on board a car ferry running across the Tsugaru-Straits in North Japan. Ship motion was measured at the navigation bridge, and a questionnaire survey was also carried out so as to obtain passengers' subjective evaluation of ride quality. This paper deals with analysis of ship motion and passengers' ride comfort. A `ride comfort index' RCI was introduced for relating human psychological response with ship motion. To conclude, the RCI combining lateral and vertical motions can be a useful index for estimating ride quality of passenger ships.