This paper reports the study on the magnetic vicosity for chain-spherical Fe UFP prepared by laser pyrolysis. Fluctuation field,anisotropy and active volume of the particles were measured and calculated. The results show that the reverse magnetizing unit is made up of n chain spheres,and equivalent to a single-domain with uniaxial particle. Variation tendency of active volume of the particles is similiar to the acicular γ-type ferric oxide particles.