The water detritiation system (WDS) of tritium plant for the International Thermonuclear Experimental Reactor (ITER) was designed. The concept of the Combined Electrolysis Catalytic Exchange (CECE) process was selected for the WDS. The design conditions are (a) tritium concentration of waste water: 3.7 × 1010∼3.7 × 1011 Bq/kg, (b) waste water flow rate: 20 kg/h (1100 mol/h), a net working rate: 300 days, annual capacity: 150 tons/year (c) tritium concentration in the H2 discharged to environment: 6.5 x 101 Bq/m3, (d) tritium concentration in the H2O vapor discharged to environment: 3.7 x 103 Bq/m3, (e) tritium concentration in the electrolyzer: ∼ 1.85 × 1013 Bq/kg. Tritium concentration in the electrolyzer is determined considering the lifetime of the electrolyzer which depends on tritium concentration. Design value of height of a unit (30cm) of water-hydrogen isotopic exchange column and the correlation between the column internal flow rates and the column diameter were determined based on similar system for Japanese advanced thermal reactor (Fugen) moderated with heavy water.