In order to estimate the effect of the dispersion of nano-carbon species into a composite electrode, ultrasonic pretreatments under various conditions have been carried out for two kinds of nano-carbons, vapor grown carbon fiber (VGCF) and acetylene black, before preparing a composite electrode with iron powder. The cyclic voltammetry behavior of the resulting Fe/nano-carbon composite electrodes is significantly influenced by the kind of nano-carbon and the pretreatment conditions in spite of as a low content of nano-carbon as 20wt.%. The largest redox current at initial cycle is obtained for Fe/VGCF composite electrode with the 30min pretreatment of VGCF in ethanol. The dispersion of nano-carbon verifies the apparent density of the composite, and then affects the feature of three-phase interface among the ion-conducting electrolyte, the redox reaction surface, and the electron conductor.