Subcritical damages sucn as Tatigue cracks ana corrosion usually occur in snip structures cturing tne service period. Such damages do not immediately cause a loss of serviceability or serious disaster such as oil spill from tankers. However, the maintenance of ship structures is very important to avoid such casualties. In this paper, the concept of the information system for damages of ship structures is proposed in order to improve the safety and to support the maintenance of ships in service. In the proposed system, all information of damages in a certain ship is managed in one database, and appropriate action could be taken based on the assessment of these damages. Three possible ways of the situation of application of the proposed system are also discussed. Secondly, the object oriented data model of crack and corrosion damages with the structural data model of a single-hull tanker is proposed. The data structure includes the information of the time when the damage is detected so that the life assessment of the damage is possible. Finally, the prototype system is constructed on the object oriented database based on the proposed data model by using the internet environment. The validity of the proposed concept and data model is illustrated by the examples of the assessment of damages such as life assessment of a fatigue crack. The validity of the proposed concept and data model is illustrated.