Sn–Fe alloy thin films were prepared by electroplating of tin on iron substrates and annealing them at 533K for 0 to 25h. The charge–discharge behavior of the films as negative electrodes for sodium secondary batteries was evaluated using a Na[FSA]–K[FSA] (FSA=bis(fluorosulfonyl)amide) ionic liquid at 363K. Although the Sn–Fe film without annealing (pure Sn film) showed a relatively high discharge capacity of 730mAh (g-Sn)−1 in the first cycle, its capacity rapidly decreased to 30mAh (g-Sn)−1 within 10 cycles. In contrast, Sn–Fe films annealed for 2 or 4h retained higher discharge capacities of 191 or 82mAh (g-Sn)−1 after 100 cycles, respectively, due to the formation of an FeSn2 alloy. Additionally, an FeSn2 film was prepared by removing the SnO layer of the Sn–Fe alloy film that was annealed for 25h, and its charge–discharge behavior was investigated.