This paper presents a method for the estimetion of transverse dispersivity in a homogeneous isotropic coastal aquifer where salt water intrusion takes place and the mixing zone of fresh-salt water is observed. An approximate solution of the concentration profile on the axis perpendicular to the fresh-salt water interface is derived from a similarity approach and compared with the concentration profile measured in a filed experiment. A two-dimensional numerical simulation on salt water intrusion and dispersion is performed using the dispersivities estimeted by the proposed method. Numerically predicted concentration profiles agree well with the experimental data.