A series of Polycrystalline spinel ferrites with general formula Cd0.4Mn0.6CoxFe2-xO4 (x=0.0, 0.125, 0.25, 0.375, 0.5, 0.75 and 1.00) were prepared by ceramic technique to study the effect of Co3+ ions substitution on their thermoelectric power, charge carrier concentration and charge carrier mobility at different temperatures. The compounds were characterized by X-ray, and IR spectra. X-ray diffraction pattern confirms the cubic single phase. Thermoelectric power values were found to be negative for all samples indicating that the majority of charge carriers are electrons. The relative magnetic permeability µ were measured as function of temperature. The phase transition temperatures were found to increase with increasing Co content.