In the previous paper¹⁾ , the motions of a 220m long and 45m wide square Plant Barge, towed from Japan to Brazil, were reported. Here in this paper, the measured results on the longitudinal strength analyzed by means of statistical analysis and spectral analysis are reported. The stress variations due to 2-nodes vertical vibration were frequently observed. These power spectra were found to have two well-defined peaks of energy, one corresponding the low frequency wave-induced stress and the other the higher frequency vibration stress. Also, these short-term probability distributions were verified to be approximated by Gaussian distribution. As for the frequency response function of vertical bending moment, the obtained values by the measured power spectra of wave height and stress were good agreement with the experimental results in model test basin. The long-term distributions of extremes on stress, estimated by following three procedures based on the measured data of short term, were fairly coincident with each other. (1) Cumulation of histograms (2) Synthesis of short-term probability distributions (3) Logarithmic normal distribution and Gumbel's extreme distribution with upper limit Moreover the investigation on the fatigue strength at the hatch corner disclosed that the damage factor very small.