The theory of limit analysis is presented for three-dimensional stability of coastal slope. In the frictional soils, the failure surface has the shape of logarithm helicoids, with outline defined by log-spirals. Dissipation rate and gravity power are obtained. By solving the energy balance equation, the expression of stability factor for coastal slope is obtained. The influences of the ratio of width and height and slope angle on the stability are evaluated. Numerical results are presented in the form of graphs. Some examples illustrate the practical use of the results.