In recent years, oil tankers which are newly built are getting large every year, and most of trial courses near the shipyards in Japan have become too shallow in relation to their dimensions. In Hitachi Shipbuilding & Engineering Co., too, the drop of speed due to the effect of shallow water has become more important, when the speed trials of large tankers are carried out at the trial course off Yuge Island. In this paper, some trial data of large tankers at the trial course off Yuge Island and some other places were analized refferring to the results of model experiment for a D. W. 47, 000T tanker, and following relations between the increase of resistance and the relative depth of sea were deduced. (1) As for the frictional resistance, α=191.4 (√ A _??_/ H -0.32) ²+16.57 (√ A _??_/ H -0.32) where, α=Rate of increse of frictional resistance in % A _??_ Area of midship section in M ² H = Depth of sea in M (2) As for the wavemaking resistance, the effect of shallow water can be neglected in this case. (3) Consequently, the upper limit of relative depth where the effect of shallow water upon the resistance of large tankers does not appear is =0.32 in ordinary speed for them. This analysis was done by Dr. Hughes' new method, and it is concluded that, in practical use, Dr. Hughes' method is better than the old one using the Schoenherr line or I. T. T. C. 1957 Model-Ship correlation line as far as the problem of frictional resistance of large tankers.