The supposed wave condition has considerable effects on the theoretically estimated extreme value of wave induced random variable of a ship. Author showed that the severity of a given wave condition is evaluated by the maximum wave height and its frequency of occurrence in each wave period interval, in the previous paper¹⁾ . In this paper, applying the evaluation method, author clarifies the severity of wave condition in winter which is considered as the roughest one in all seasonal wave conditions. It is found out by observation of wave statistics in the North Atlantic by Walden³⁾ , those by Hogben and Lumb⁴⁾ (sea area 1, 2, 6 and 7), and those in the North Pacific by 80 th Committee of the Japan Ship Research Association⁵⁾ (sea zone 3), that at longer wave period intervals the highest wave in year is observed in winter in almost cases but at shorter wave period intervals the one is observed in other season than winter in some cases. Accordingly, though the extrem value of the variable estimated by supposing wave condition in winter instead of that in all seasons is larger at most about 10% than the one in all seasons generally, sometimes the value is less than the latter. The population of the variable in all seasons is total sum of the population of it in each season. As it is expected that the population in winter includes the extreme value in all seasons which is to be regarded as the exact value for a ship, the value in winter is compared with that in all seasons. The relation between these two values varies according to the wave period T _max at which the maximum standard deviation of short-term distribution of the variable is observed. In the case where T _max is over 5 sec the former is nearly equal to the latter in most cases, but sometimes the former is less than the latter considerably where T _max is from 5 to 9 sec. Similar relation must be found between the extreme value predicted by the result of long-term full scale measuement of the variable in winter and the exact value. The effect of sample size is also to be considered in this case, because the period of full scale test is usually shorter than the observation period of such wave statistics described above. It is concluded by my study that the value may be nearly equal to the exact value where T _max is from 7 to 13 sec and may be considerably less than the exact one in other range of T _max. This means that the correct extreme value of wave bending moment may be predicted by full scale measurement where length of ship is from 120 to 400 m (cargo ship) or from 100 to 320 m (tanker), but to predict the value by this method may be under estimation where ship length is out of the ranges. The method of seasonal wave observation to get more correct data for estimation of the extreme value than the wave statistics in winter is studied. It is made clear by analysis of above wave data, that the value of the variable in winter and autumn in the North Atlantic or in winter and spring in the North Pacific agrees with that in all seasons accurately even if T _max is in the range from 5 to 9 sec. This is an useful guide of wave observation when continuous measurement is not allowed.