For antennas that are mounted on the same object, coupling through surface waves typically is very significant. The accurate characterization of this coupling is instrumental to the design of antennas with required isolation between its multiple functions. When the antennas consist of only conducting elements, they can be efficiently modeled using the method of moments (MoM). However, most MoM programs developed so far are not suitable to the accurate calculation of mutual coupling. For example, MoM programs based on wire-grid models do not accurately model surface-wave coupling. Furthermore, the MoM yields a full matrix equation, the solution of which is very memory intensive and extremely time consuming for a large scale modeling problem when the number of unknown is large. This is because both the memory requirements and the CPU time per matrix vector multiply are proportional to N/sup 2/. To overcome these problems, we considered the application of the multilevel fast multipole algorithm (MLFMA) to reduce the memory requirements and computational complexity per matrix-vector multiply to O(NlogN) (Chew et al. 1997, and Chao 1998). Our preliminary investigation shows that the MLFMA can predict accurately the S-parameters that characterize the mutual coupling/isolation between antennas. It is a useful tool for the design and performance evaluation of multi-antenna systems. We consider two antenna configurations to demonstrate the accuracy of our algorithms. The first configuration consists of two inverted-L antennas on a finite ground plane and the second composes two loop antennas, also on a finite ground plane.