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Different iron carbides were synthesized from the iron oxalate precursor by varying the CO carburization temperature between 320 and 450 °C. These iron carbides were applied to the high-temperature Fischer–Tropsch synthesis (FTS) without in situ activation treatment directly. The iron oxalate as a precursor was prepared using a solid-state reaction treatment at room temperature. Pure Fe₅C₂ was formed at a carburization temperature of 320 °C, whereas pure Fe₃C was formed at 450 °C. Interestingly, at intermediate carburization temperatures (350–375 °C), these two phases coexisted at the same time although in different proportions, and 360 °C was the transition temperature at which the iron carbide phase transformed from the Fe₅C₂ phase to the Fe₃C phase. The results showed that CO conversions and products selectivity were affected by both the iron carbide phases and the surface carbon layer. CO conversion was higher (75–96%) when Fe₅C₂ was the dominant iron carbide. The selectivity to C₅⁺ products was higher when Fe₃C was alone, while the light olefins selectivity was higher when the two components (Fe₅C₂ and Fe₃C phases) co-existed, but the quantity of Fe₃C was small.