The fast multipole method is applied for calculating the radar cross section of a groove in a perfectly conducting plane. The coupled integral equations for the induced currents of a groove in a perfect conducting plane are obtained in terms of equivalent principle, and solved by using CGM and current iteration method, FMM is employed to speed up the matrix-vector multiplication. After FMM acceleration, both the computing time and memory needs are reduced from O(N²) to O(N^1.5) without increasing the complexity of implementation. Some examples are calculated, the numerical results are in perfect agreement with the MOM result.