It seems to be a very difficult problem to calculate the natural frequency of fore-and-aft vibration of superstructure in the early stage of designing, because there are many factors affect the vibration characteristics. The authors had presented an estimation method of its natural frequency by the use of an electronic computer in their previous paper, and this time they present some research works they made after the previous paper. That is : 1) In order to consider the accuracy of the above mentioned method, comparisons are made between the calculated natural frequency by the method and the observed one obtained from full-scale vibration measurements carried out on a lot of newly-built vessels. 2) An empirical estimation formula of the natural frequency available in the early stage of designing is derived from those comparisons. 3) Effects of corrugated plate used for the side wall of superstructure on the natural frequency and the actual vibration behavior of superstructure being on a voyage are investigated.