Bench-scale experiments using iron modified and unmodified photocatalysts (Fe/TiO2 and TiO2) were conducted to compare their decomposition efficiencies with formaldehyde. The effects of operating parameters on the decomposition efficiency were further investigated. The grain size of iron doped photocatalysts ranged from 25 to 60 nm. The iron doped content of 1, 3, and 5% Fe/TiO2 photocatalysts was measured as 1.2, 3.1, and 4.7%, respectively. The UV-visible analytical results showed that a significant red shift was observed while the iron doping content of Fe/TiO2 increased from 0 to 5%. Two continuous-flow reaction systems, the ozonolytic and the photocatalytic reactors, were combined in series to investigate their capability to decompose formaldehyde by Fe/TiO2 photocatalysts with six operating parameters, namely, the influent formaldehyde concentrations (0.15, 0.30, and 0.45 ppm), the relative humidity (5, 35, and 55%), irradiation by light (visible, near-UV, and UV), the reaction temperatures (25, 30, and 35°C), the iron doping content (1, 3, and 5% Fe/TiO2), and the injected-ozone concentrations (2,000 and 3,000 ppb). The optimal operating parameters obtained in this study were an influent formaldehyde concentration of 0.15 ppm, a relative humidity of 5%, irradiation by UV light, a reaction temperature of 35°C, an iron doping content of 5% Fe/TiO2, and an injected-ozone concentration of 3,000 ppb. Overall, the efficiencies of different decomposition techniques for formaldehyde followed the sequence: UV/TiO2 + O3 > O3 + UV/TiO2 > UV/O3 ≈ O3. A maximum formaldehyde decomposition efficiency of 92% was obtained by using the UV/TiO2 + O3 technology.