From investigating the characters of mangrove forest related to wave fender capability includes: crown diameter, diameter at breast height, stand height, forest cover and density and defining wave heights at different locations includes: in front of forest, 20, 40, 60, 120 metric way can be seen that: wave height reduction in mangrove forest obeys the exponential function: hs = a*e[b..]. Using SPSS software to define coefficients a and b of the equation. From that, the equation of reducing wave in mangrove forest were defined. Through the characteristics of mangrove forest structure, the structure coefficients of each mangrove forest type in Vietnam have beeb established. Structure coefficient is the main basis to define the width of mangrove forests and reduce waves to safe level after going through mangrove forest band (wave height is considered safe if it is equal to or lower than 30 cm in the maximum of 500 cm).