A computer code, MIG, has been developed to interface the magnet design and the three dimensional Monte Carlo code MCNP to perform neutronics design analyses. MIG prepares all the required MCNP cells and surfaces to simulate the magnets described in EFFI input. Extra zones with different materials could be added to envelop or divide the winding packs of the magnets. Examples of the input and output of MIG used by MCNP are given to illustrate the different capabilities of MIG.