A 220m long and 45m wide square Plant Barge was towed for about three months from Japan to Brazil covering the distance of 13, 000 sea miles. During this towing, some of the items concering motions and strength of the abrge were measured for 40 minutes per day. Observations of environmental conditions were made simultaneously, and, in some cases, wave heigths were measured using throw-in type wave meters. The measured results were analyzed by means of statistical analysis and spectral analysis. The feature of short term responses was investigated, and the spectral form thus obtained was compared with that of Pierson-Moskowiz type wave spectrum. As for the frequency response function of motions, the calculated values obtained by using the measured power spectra of wave heigth were compared with the measured ones and experimental results in model test basin, and a good agreement was found between the measured values and experimental results, and also the calculated values indicated to agree fairly well with the measured ones.