This paper describes the results of a series of expriments on the effectiveness of the longitudinal bulkhead which have been carried out with five simplified ship models. These models are fitted with longitudinal bulkheads of different lengths, number of rows, stiffening of bulkhead plates, end construction etc. From these experiments, the following conclusions have been obtained : (1) The shear-lag effect is remarkable only within the range approximately equal to 1. 2 times the depth of the longitudinal bulkhead from the bulkhead end. The remaining middle part is fully effective and the distribution of longitudinal bending stresses on the ship in this region can be-obtained from beam theory. (2) Maximum shearing stresses in side shells and bulkhead plates near the end of bulkhead. are considerably larger than those of the middle part.