Ship-based measurements of wind profiles in the boundary layer were carried out suc-cessfully in the tropical western Pacific, as a part of the observational program of the MONEX in Japan. A wind-finding procedure by use of pilot-balloon observation on board a ship is developed for the present study. Observed wind field in the boundary layer shows the characteristics of the mixed layer, that is, the layer of uniform wind speed and direction in the vertical. The depth of the layer almost corresponds with that of the mixed layer deter-mined from the thermodynamical structure of the boundary layer. The results seem to be in contrast with the general feature in the undisturbed trade winds where there appears appreciable vertical shear of wind speed in the thermodynamically determined mixed layer. A few estimations of cross-isobar angle and ratio of actual to geostrophic wind speed of the boundary layer wind are made, though roughly, in good agreement with climatological value in lower latitudes. An evidence is obtained that the depth of "a density current" by downdraft in front of the squall cell is about two hundred meters above the sea surface.