Abstract: In this paper, the magnetic behaviour of Fe-Ni invar alloy is analysed on the basis of Mossbauer spectrum. The coexistence of two magnetic states of iron atom is revealed and invar mechanism of Fe-Ni invar alloys is interpreted according to the above view point. The equations with which one can determine experimentally the magnetic moments of the iron atom in two magnetic states are presented. Anomalous phenomena of Fe-Ni alloy at about 32 wt% Ni are observed.