Abstract: The effective multiplication factors and fluence or currents of five 1D typical fast critical benchmark experiment models and one 3D fast critical benchmark experiment model were calculated using the 3D multigroup Monte-Carlo code MCMG. The results were compared with those of multigroup S_N code ANISN and continued energy cross section Monte-Carlo code MCNP, respectively. The results of MCMG are in good agreement with those of ANISN, and close to those of MCNP as well. It is believed that MCMG can be used to test the validation of multigroup cross sections.