Nowadays, marine structures such as floating breakwaters, floating berths, floating platforms, etc. are used widely. Floating structures have some advantages such as more compatibility with marine environment, easier and faster transportation for temporary works and capability to install at any depth. The most commonly used type of floating breakwaters are rectangular ones as they have simple and rapid installation and also their deck can be used. For behavior study of floating breakwater, mostly structural response and transmission coefficient are considered. This article investigates effect of adding two thin boards vertically to the sides of rectangular floating breakwater to convert it to Π shaped floating breakwater and other geometric dimensions on the transmission coefficient of floating breakwaters. The results show that adding side boards and, mooring and fixing floating breakwater may reduce transmission coefficient considerably. In addition, dimensional aspects of can effect significantly on behavior of floating breakwater.