Along with the growing tendency of constructing ships larger than 65, 000 D. W. T. and/or 100, 000 D. W. T., doubling system ought to be adopted in order to maintain the longitudinal strength of the ship for structures such as upper deck and bottom shell plating. We investigated the nature of cracks at the root of one side full penetration weld joints which are inevitable in doubling system. The summaries are as follows : (1) Root cracking of one side full penetration welds may be divided into two kinds, notably, cracking in weld metal and cracking in heat affected zone. (2) Cracking in weld metal originates in the root notch and grows along the grain boundary in dendritic structure. The cause is considered to be the stress concentration at root notch due to shrinkage. (3) Cracking in heat affected zone originates in the root notch. This type of cracking is transcrystalline. It is conceivable to be due to the so-called cold crack. (4) In order to minimize the root cracking in weld metal, fillet type welding process would be effective. (5) To minimize the root cracking in heat affected zone, low hydrogen type electrode would be preferable following preheating at the temperature of more than 750°C.