The manoeuvring characteristics of a ship at very large drift angles will be significantly influenced by non-linear hydrodynamic forces, initiated by the cross flow underneath a ship from the inflow to the leeside. It is considered that the magnitude of cross flow drag is very sensitive to the forms of the local cross-sections of a ship, such as thickness of sections and smoothness of the edge. This paper dealt with the cross sectional aspect ratio and radius of round edge effects on cross flow drag acting on rectangular cross-sections, which are theoretically investigated by using of vortex model developed by the authors. From the results of numerical calculations, cross flow drag acting on rectangular cross-sections decrease abruptly as increasing corner radius, especially for the sections with a breadth to height ratio becoming larger. Thus, the cross sectional aspect ratio and radius of round edge effects will be important factors for cross flow drag acting on rectangular cross-sections, and therefore these effects should be also considered in evaluating the local cross flow drag of a ship moving at very large drift angles.