Along with the recent tendency to the increase in size of oil tankers, the scantlings of hull structural members, especially primary supporting members, have become very large. Also, these members are subjected to great loads, and a considerably high stress may occur locally if the scantlings of the structure are not sufficient. As one of the series of studies to cope with the increase in size of oil tankers, the authors try to investigate the strength of horizontal girders of a 120, 000 DWT tanker by comparing the stress measured on an actual ship with the stress calculated by an electronic computer. The results of stress analysis coincide considerably well with the stress measured on an actual ships, showing that the calculation method described below is fairly good. This paper also discusses the strength of horizontal girders in various loading conditions, the effectiveness of center line vertical web and the effect of transverse rings neighboring watertight bulkheads on horizontal girders.