For the design of ships or offshore structures, a reliable database of long-term wave is very import. Nowadays this valuable information is mainly available in three forms : (1) based on visual observation (ship data); (2) hindcast data, such as provided by Ship Research Institute (SRI) and (3) based on wind data, such as provided by Global Wave Statistics (GWS) system developed by N. Hogben et al . in UK. Those different approaches to create the statistical database presents significant differences for the same sea area subdivision and the designer must choose the convenient database for each area. In this work, the long-term prediction and mutual transformation were considered to give the basic information aiming at the unification of those different databases.