RO-RO passenger vessel has a wide non-separated car deck. Once free flooded water is piled up on it, the large heel moment could be the cause of capsize because of this feature. The stability standard of RO-RO passenger vessels was deliberated at IMO commission from 1994 to 1995 in order to prevent capsizing disaster like the one of ESTONIA in 1994. The authors, one of which was a member of the IMO expert panel, conducted an experiment on the stability of this type of ship because few papers have been published on this problem. The experiment was carried out in beam seas using a model ship with a side damage hole which is prescribed by the SOLAS regulation. After the duration time of the experiment (30 minutes in ship scale) in irregular waves the ship survived with a constant mean heel angle φ₀ and a mean water volume on deck w in most test conditions, but capsized in a few ones. The variation of these constant values with CG height, existence of center casing, height of freeboard and initial heel was discussed. It was clarified that φ₀ tends to decrease and w tends to increase as GM_d (GM in damaged condition) have a larger value, and that ship can survive even with w of 40% of intact ship displacement if she has a large (not extraordinary) GM_d value. The effect of resonance of roll motion on this problem was also studied based on the test result in regular waves. φ₀, w and some other data often have a peak near the resonant frequency, so the stability test of RO-RO passenger vessel should be carried out including resonant conditions. The height of water on deck above the calm sea surface H_d was proposed as an index which settles the balancing condition. It was clarified that Hd keeps a certain value above calm outer surface when wave hight is not so low and mean heel angle is not so large to lee side. The equilibrium curve for each GM_d can be calculated from GZ -curves with a constant volume of water on deck, fully static calculation, and figured on H_d -φ₀ diagram. It is concluded that the possibility of capsize can be judged by this equilibrium curve.