The CoFe2O4 ferrofluid was studied by positron annihilation technique. Theexperimental results showed that there were three kinds of lif e-time stateτ1= 151.0ps
τ2 = 457.9ps and τ3 = 2325.6ps in the positron annihilation spectrum. Itindicated that there were a large number of vacancy and vacancy cluster in thesurface layer of the CoFe2O4 nanoparticles.