The influence of wave steepness on force coefficient for large square and rectangular section caissons has been studied experimentally and compared with the results obtained using the modified linear diffraction theory proposed by Mogridge and Jamieson. The square caisson was tested for orientations of 0°, 15°, 30° and 45° of its side with the wave front and the rectangular caisson was tested for two aspect ratios of 3/4 and 4/3 for normal incidence of waves on its side.