Grass covers have been applied as an effective measure for protecting river levees and sea dikes. We conducted experiments to show how roots considerably improve the shear strength of soil on dike slopes. Roots of 1-year-old Bermuda and Carpet grass may increase the total shear strength of up to 20 kPa. Exposed to severe overtopping flow, dike slopes may possibly fail in various manners including ‘head-cut’, ‘roll-up’ and ‘collapse’. The ratio between shear strength of the grass cover and its subsoil layer would get a value of two to distinguish the first two manners and would be zero for the last one. To some extent, the findings contribute to the basis for thoughtfully investigating the strength and failure mechanism of grass-covered slopes.@en