The origin of Fe oxide deposition on zirconium oxide with UV irradiation has been investigated in this study. After 7 day corrosion in the flowing autoclave, Fe based oxide is formed on the zirconium oxidewith UV irradiation at 260°C, 6 MPa DI water. Zircaloy-4 coupon is irradiated with a 200 mW·cm−2 UV, and the dissolved oxygen level is maintained below 100 ppb, and dissolved hydrogen concentration is maintained as 2.5 ppm. Zircaloy-4 coupon supplied from Westinghouse is used for this study. MULTEQ version 4.0 developed by EPRI is adopted to simulate how ions dissolved in water can generate deposits on the zirconium oxide with UV irradiation. ICP-OES data after 30 d corrosion in the flowing loop experiment is used for input file for MULTEQ simulation. The system temperature is set as 260°C, and 2,592 L of water is considered the total amount water into the autoclave (0.06 mL·min−1, 30 d). Total numbers of simulation run is set as 8, and the system pH at 260°C is 6.06. Oxidation potential after run #8 is −0.44 V. From MULTEQ simulation, most Fe is existed as Fe(OH)3 and Fe(OH)2, and Fe ions can also exist, but no Fe metal observed. 5.09 × 10−6 ppm (9.73 ppb) of Fe2+, 2.81 × 10−6 ppm FeOH+, and 3.77 × 10−9 ppm Fe(OH)3are in the system. It can be concluded Fe is existed as ion or hydroxide form in the solution. Two precipitates are found from MULTEQ simulation, First, NiO(s) = 5.21 × 10−5 g (52.1 μg), NiFe2O4 = 8.06 × 10−5 g (80.6 μg), and still they are negligible amount. The total concentration of Fe in the electrolyte is the summation of each Fe species concentration and it is equal to 2.69×10−4 ppm. This value is equivalent to 0.269 μg·kg−1 in the solution. The total water volume of the 30 d experiment is 2,592 L (considering water flow from high-pressure pump), so the amount of Fe from ICP-OES data and MULTEQ results in 2,592 L electrolyte is 697.2 μg. This value is order of magnitudes higher than the mass of Fe from the deposits, which was already an upper estimate based on the assumptions. This clearly shows that Fe ions dissolved in the electrolyte can be the source of Fe3O4 on Zr oxide during corrosion with UV irradiation.