It is considered of a fundamental importance in safe, rational and optimum design of ship structures to grasp the true ultimate strength of each structural component of the primary members. In fact, it has often been experienced in structural model tests on deep girders that the ultimate collapse load of the structures is definitely influenced by either local buckling or by the lateral instability of both compression flange and web. This means that the ultimate collapse load of such deep girder structures would be unsafely lowered by these effects than those expected by the plastic analysis on the structure, unless some suitable considerations are made in determining the scantlings of the web and the flange as well as in the arrangement of the stiffeners, tripping brackets, etc. In this paper, the ultimate strength of deep girders is investigated from view point of the safe design against the lateral instability of these members. First, theoretical analysis is made on a deep girder subjected to bending moment, M (around the strong-axis of the girder) together with the axial thrust, N acting in the plane of the web. Analysis based on the flow theory of plasticity is performed by considering the elasto-plastic behavior of the flange and web including the effect of strain reversal, and the successive step by step method is applied to find out the maximum bending moment, M _max which the girder can carry at the ultimate state. Computation program is developed for the analysis, and the numerical calculations are made by means of electronic computer, in order to investigate the effect of the axial thrust, the span of the girders, the cross sectional properties (flange breadth b , web depth d , etc.), rigidity of the web stiffeners and the magnitude of initial deflections on the ultimate strength of deep girders. Experimental tests are also carried out on model girders with various length of span, in which tested are eight specimens without stiffeners and twelve specimens with web stiffeners of various breadth subjected to a uniform bending moment. Simultaneous measurements and recordings are made on the lateral deflections of flange and web as well as on the strain distributions in the components so as to determine the equivalent length of the span S . Comparisons are made between the test results and the theoretical predictions on the ultimate strength of the girders, and a fairly close agreement is found with each other. Then a survey is made on the conventional scantlings of center girders, transverses, horizontal girders and longitudinals of large tankers built in the past years to examine the ultimate strength against lateral instability of these members. An approximate formula, b=A√Sd is suggested for the design of the deep girders with the minimum breadth of the flange necessary for assuring their ultimate strength up to a certain prescribed value of the ultimate strength factor, k = M _max/ M_p ( M_p being the fully plastic moment of the girders), where the coefficient A is determined by giving the k -value and a parameter η, which depends upon the bending rigidity of the stiffeners on the web.