An analytical solution is derived for laminar boundary layer under cnoidal wave motion. The solution is expressed in terms of Fourier series expansion of simple functions. Thus, computation of the velocity and the bottom shear stress can be made very easily. According to detailed analysis of the solution, quantities such as velocity, bottom shear stress, boundary layer thickness and phase difference show considerable difference from those under sinusoidal wave boundary layer. Comparison with present and existing experimetal data sets verifies the validity of the theory.