The effects of molecular weight and solubility of carboxymethyl cellulose (CMC) on the electrochemical characteristics of zinc plate electrodes and Zn-Ni batteries were investigated with and without ZnO in 6 M KOH solution, respectively. The result of electrochemical impedance spectroscopy (EIS) indicated that as an important additive in zinc paste electrodes, CMCs with M.W.s of 700,000, 250,000, and 90,000 can all suppress the corrosion of zinc. The zinc deposition experiment revealed that the CMCs, especially the soluble M.W. 700,000 CMC, can suppress zinc electrodeposition kinetics and decreases the porosity of zinc deposits. Zn-Ni battery performance tests indicate that a suitable concentration of soluble CMC will decrease charge and discharge polarization and increase discharge capacity. These results indicated that various M.W.s and the solubility of CMC in 6 M KOH showed complex electrochemical effects on zinc electrodes in alkaline solution, and their addition to a zinc paste electrode needs careful consideration.