Iron carbide nanoparticles have long been considered to have great potential in new energy conversion, nanomagnets, and nanomedicines. However, the conventional relatively harsh synthetic conditions of iron carbide hindered its wide applications. In this article, we present a facile wet-chemical route for the synthesis of Hagg iron carbide (Fe5C2) nanoparticles, in which bromide was found to be the key inducing agent for the conversion of Fe(CO)(5) to Fe5C2 in the synthetic process. Furthermore, the as-synthesized Fe5C2 nanoparticles were applied in the Fischer-Tropsch synthesis (FTS) and exhibited intrinsic catalytic activity in FTS, demonstrating that Fe5C2 is an active phase for FTS. Compared with a conventional reduced-hematite catalyst, the Fe5C2 nanoparticles showed enhanced catalytic performance in terms of CO conversion and product selectivity.