The landware slopes of sea dikes in the north of Vietnam are steep and solely constructed by soil covered with grass due to the budget constraints and also the out-of-date design guidelines.The resistance against erosion due to wave overtopping of several grass covered slopes were tested with the Wave Overtopping Simulator to show if a slope covered with grass could withstand a certain amount of overtopping. An analysis of all test results obtained in the Netherlands and Viet Nam is made as an attempt to establish a reference for empirically assessing the strength of grass covers. Slope classification should be based on its specifications whether it is a plain slope or a slope with existing damages and obstacles.