As a method for separating tritiated water from radioactively contaminated water, an additive was mixed into the contaminated water and their treatments of agitation/circulation and electrolytic was considered to improve of the separation efficiency. Carbide powder and ore (silica stone) powder were used as additives. The radioactivity concentration of the tritium-contaminated water (tritiated water; HTO) was 366 Bq/mL before treatment, however it obtained 333 Bq/mL and decreasing rate of 9.02%, and a high separation efficiency after treatment. Furthermore, in the reproducibility experiments (five times) using high content of HTO, the average HTO/H2O separation efficiency was obtained about 5.59% indicating good reproducibility. The agitation/circulation treatment process was dissociated and ionized HTO, and was prepared colloidal particles by OT− and 3T+. In the electrolytic treatment process, the colloidal particles in HTO were deposited on the both electrodes applied DC voltage, and was efficiently removed tritium. These treatment processes using additives were found to be useful for removal of tritium.