A nonlinear theory is developed in this paper to analyze the drift force on a rectangular body close the free surface. The overall problem is divided into three parts wihich are an external outer region, an internal outer region and an inner region. Solutuins of these problems are matched each other by means of the matched asymptotic expansion procedure and the solution is harmonic in time. This theory satisfies the conservation of the wave energy. Some numerical results show that the negative drift force occurs at a certain frequency. Some experimental results also show the possibility of the negative drift force.
