Nowadays, more than half of the world population live in coastal regions, and coastal areas are vital with many economic benefits. Breakwaters are commonly used to protect harbors and coasts from wave attack.Traditional bottom-sitting breakwaters can effectively fulfill the demands of harbor protection in places where water is relatively shallow. However, the heavy traffic and large ship tonnage due to rapidly-developed international trade and maritime traortation are demanding much deeper water depth in harbors. As a result, new harbors are extending towards the ocean and traditional bottom-sitting breakwaters are no longer suitable economically. There is a need for new types of breakwaters that can effectively and economically be deployed in places where water is deep or bottom foundation is weak. Floating breakwaters provide numerous advantages over bottom-sitting breakwaters including better compatibility with the environment, mobility and easier installation, lower manufacture and construction costs and enhanced applicability in deep waters and in areas with loose sea bed conditions. This kind of structure despite of all its advantages can’t dissipate all the sea wave energy. So the designers are trying to increase the efficiency of this kind of structures. The purpose of this experimental study is to investigate the effects of installing porous plates under the floating breakwater on breakwater efficiency, mooring forces and structure movement. In this experimental study a trapezoid shape floating breakwater with the slope of 60 degree has been employed. The proposed modification on this breakwater includes installing three porous plates with different porosities. In order to moor the structure, four galvanized steel cables by diameter of 3 mm were employed. The experiments were conducted in the wave flume at the Subsea R D Center of Isfahan University of Technology. The wave flume of Subsea R D Center has a length, width, and height of 108, 3, and 2.2 meters, respectively. The wedge type wave generator was used to generate regular wave with the period of 0.9 s to 1.45 s. The water surface level was measured using wave gauge installed in the wave flume. Also one S-type load cell capable of measuring tensile strength, were used to record the mooring line. Experimental results of this study have shown that installing porous plates underneath the breakwater enhances significantly the efficiency of the structure in attenuating the incident waves; but increases the mooring line forces. It has also been found that the damping in heave direction can be enhanced by using the porous plates. In this way the heave motion will be reduced while the sway motion will increase. Based on the test results, empirical expression was formulated to describe the Ct for different breakwater drafts under regular waves. Key word Pontoon, Floating Structure, Porous Plates, Wave, Mooring Force, Heave, Sway, Load cell.