To investigate the effect of photochemical aging on the stable carbon isotopic ratio (δ¹³C) of oxalic acid (OxA), a dominant organic species in atmospheric aerosols, we conducted a laboratory photolysis of OxA under H₂O₂‐Fe³⁺(Fe²⁺)‐UV system in aqueous phase and measuredδ¹³C of remaining OxA. Our results showed that a significant photolysis of OxA occurred with OH radical but the isotopic fractionation of OxA was insignificant. In contrast, in the presence of Fe³⁺ (Fe²⁺), we found a significant enrichment of ¹³C in remaining OxA. We also found that kinetic isotope effect (KIE) of OxA largely depends on photochemical age (irradiation time) and concentration ratios of OxA to iron; 3.20 ± 0.49‰ (2.18 ± 1.18‰) and 21.62 ± 5.41‰ in 90 min and 180 min irradiation, in which OxA and Fe³⁺ (Fe²⁺) ratios were 50:1 and 200:1, respectively. The enrichment of ¹³C in remaining OxA was more significant during the photolysis catalyzed by Fe³⁺ (7‰) than by Fe²⁺ (3‰) in 90 min irradiation when OxA and iron ratios are the same (50:1). This study provides a laboratory evidence for the isotopic enrichment of ¹³C in OxA with photochemical aging. This approach is useful for better interpretation of atmospheric isotopic measurements in terms of the extent of atmospheric processing of aerosols.