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In this research we examined the sensitivity of micronucleus assay for monitoring clastogenic effects of low dose fast neutrons. Syrian mice (12 weeks old) were irradiated by fast neutrons emitted from a 241Am-9Be source. The absorbed dose was 1.5, 2.25, 3.375 and 5.06 cGy. Mice were scarified by cervical dislocation at different post irradiation times (24, 48 and 72 h). The results obtained show that the frequency of neutron-induced micronuclei in polychromatic erythrocytes (PCES) is significantly higher than those of control groups (P<0.05) at neutron dose used in this experiment. We concluded that micronucleus assay is an effective and also inexpensive method for monitoring clastogenic effects of high LET radiation in low dose levels.