Floating breakwater is simulated in the time-domain to evaluate wave blocking. The open-source CFD library OpenFoam has been used. It contains a method for solving free surfaceNewtonian flows using the Reynolds averaged Navier–Stokes equations coupled with a volume of fluid method. At first a boxshapedfloating breakwater is simulated and the result is verified by experimental tests. Also, a floating breakwater with two airchambers is simulated and its performance is compared with boxshaped breakwater. The results show air chambers can improve the hydrodynamic performance significantly. Besides, inserting nozzle for air chambers improve the results by 15-25%.