Little is known about the strength of the land-side slopes of the Vietnamese sea dikes under overtopping attack. Some grass-covered slopes were tested by a wave overtopping simulator. The paper describes the simulator and presents the test results. The tested grass covers could withstand overtopping rates varying from 20 l/s to 100 l/s per meter of dike length in some hours. Damage usually started at bare spots, at the transition between the slope and the toe, at the transition between different materials, and around objects (e.g. big trees). These features reduce the strength of a grass cover and therefore should be avoided.