The systematic variation in line width of ferromagnetic resonance with the Fe content was observed at X band (9.78GHz) in (Ni0.5Zn0.5)1−xFe2+xO4 (-0.2 ≤ x ≤ 0.2). The line width of the stoichiometric composition (x = 0) showed minimum value, 50 Oe. In contrast, the line width of the non-stoichiometric compositions sharply increased to 210 Oe with increasing nonstoichiometry (x). The mechanism for this line width broadening was investigated using thermoelectric power and electrical resistivity, since the contribution of anisotropy and porosity to the line width was negligible in this compositional region. In Fe excess region, Fe2+ ion concentration increased with increasing Fe content, resulting in line width broadening due to relaxation. But, it was suggested that Ni3+ and Fe2+ ions coexist in Fe deficient region. Therefore the increase of line width in nickel-zinc ferrites originated from the Fe2+ /Fe3+ magnetic relaxation in Fe excess region, and the Fe2+/Fe3+, Ni2+/Ni3+ magnetic relaxation in Fe deficient region.