Abstract: Based upon regular wave experimental data and taking into consideration the boundary and continuity conditions which must be satisfied in wave diffraction,we obtained empirical formulas for computing diffraction coefficients of regular waves passing a semi-infinite single breakwater.From wave spectrum theory,principally taking into account the angular spectrum of irregular waves,we further derived analytical formulas to compute diffraction coefficients of irregular waves trarelling around a semi-infinite single breakwater.The calculated values of diffraction coefficients have been compared with measured data.Preliminary result of comparison indecated that generally they are in good agreement.The formulas proposed in this paper can thus be applied to the designing breakwatewaters.For the purpose of rapid computation of the approximate values of diffraction coefficients in the preliminary stage of design,a series of tables and diagrams have been prepared using the proposed formulas,Tables and diagrams can be used equivalently to each other.