The major subject of the present report is concerned with the deformation of ship's bottom caused by welding. And it will be shown that the final shape of bottom on berth-lifting at ship's ends and shrinking fore & aft-is determined by the welding to the bottom and its vicinity, and not the main deck and above. It will also be shown that the intensity of tensile reaction stress in main deck plate of the large tanker does not increase with the progress of welding work through construction period. It follows from the above that the apparent sagging moment by which the weld-constructed ship tends to sag after her launching is not due to the welding internal stress which may be considered to be restricted on berth and released at water born, but the differences between the distribution of ship's own weight and reaction force by keel block and shore.