Since the tragedy of Estonia happened in Baltic Sea, the enhancement of safety of RO-RO passenger ship is the interested problem in the world. Therefore, many research works have been continued in many countries, including our country. There was a proposal by United Kingdom for International Maritime Organization, that is the requirement of the stability performance related to free water effect on deck. For investigating the proposal, a model ship was made as a typical RO-RO passenger ship in Japan and model experiments were carried out in damaged condition and in irregular beam seas with JONSWAP spectrum. The main conclusions from the experimental results are shown as follows, (1) The ship does not capsize in the condition of no initial heel, but she capsizes with initial heel to damage side in higher significant wave height than the proposed one. (2) The critical significant wave height is affected by the peak period of the wave spectrum. (3) The positive area in GZ curve is related to the critical significant wave height.