Growth behavior of the intermetallic compound (IMC), FeSn2, was investigated in the liquid Sn/solid Fe reaction couple over the annealing temperatures from 250 °C to 400 °C. Low-carbon steel AISI 1018 was chosen to make Fe samples. The morphology and thickness of the IMC formed between Sn and Fe were examined using scanning electron microscopy (SEM). In addition, energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) were used to confirm that the IMC is FeSn2. The growth kinetics of FeSn2 was modeled by parabolic law and empirical power law. Based on the models, the growth constants, the activation energy, and the time exponents were established at different annealing temperatures. It was found that the time exponent values obtained by fitting with empirical power law deviate from 0.5, meaning that volume (bulk) diffusion is not the only rate-controlling process in the liquid Sn/solid Fe reaction couple. Also, a variation in the time exponent values is indicative that the growth behavior is correlated with grain size growth and irregular grain morphology at different annealing stages. The results of this research show that AISI 1018 steel can readily react with Sn to form IMC on the interface. This is an essential requirement of soldering action using Sn-rich solders.