Abstract: Free!ship as one ship design tool, has been widely utilized due to its robust functions, userfriendly interface and available free use. As for the work of preliminary ship design, it is desirable that it can determin e the ship＇s principle dimension prior to the completion of ship lines design. Free! ship can not meet this requirement, however, an effort was made to re-develop it on the basis of its original functions and perfect reproduction environment. For this purpose, flexible oil tanker was studied as an example and two modules (i.e. principle dimension regression module and dimension optimization module) were added into the Free! ship p by programming with Delphi to determine the principle dimensions. The development method and procedure are established and they provide a basis for further development and research.