The second-order potentials, pressures and forces are calculated by the Boundary Integral Equation Method, which was proposed in previous report (1), in two-dimensional radiation problems. The second-order forces are decomposed into each non-linear terms and the quantitative comparison is made and dismissed. The experiments in forced heave and sway motions are carried out and compared with the calculations. Results obtained are as follows : (1) The numerical results by the present method agree fairly well with existing results and experimental results in first and second-order radiation problems. (2) In radiation problems, the non-linearity of the body-boundary condition is dominant and the second-order forces are nearly proportional to the wave-number of the motion. (3) Total second-order forces show good coincidence with the experimental results.