The present paper is concerned with the free surface integral for a calculation of the nonlinear wave forces on plural vertical cylinder with arbitrary cross-sections and with the effects of nonlinear free surface on wave forces. The modified Laguerre integration method was applied and its convergence was confirmed in the cases of a circular, a rectangular and twin circular cylinders. Further, the present method was compared with the method given by Eatock-Tayler, and the computational times and those properties were investigated. The numerical results for maximum wave forces on the cylinders obtained by the present method were compared with the experimental results, so that the effects of nonlinear free surface on wave forces were clarified.