The formation of intermetallic compounds (IMCs) during the friction stir welding (FSW) of aluminum and steel is problematic because these IMCs can reduce weld strength. In this study, the mechanism behind the observed rapid growth of IMCs during the dissimilar FSW of aluminum and steel was investigated. The temperature during welding was measured using K-type thermocouples, and the microstructures of cross sections of the welded materials were examined via scanning electron microscopy. Microstructural observations indicated that the growth of IMCs was not constant, but occurred in two rapid growth steps. The first phase of rapid IMC growth was observed immediately after the probe contacted the steel, while the second began in the region subjected to the large downward pressure of the tool shoulder on the steel plate. The measurements showed that the temperature underneath the tool shoulder was higher than that at the tool probe. Additionally, it was found that the two IMC growth steps and the growth rate could be expressed by an equation based on metallic diffusion and the measured temperatures. As the IMCs grew rapidly via contact between the steel plate and the tool probe or shoulder, it is necessary to control such contacts to inhibit IMC growth. This strategy and the proposed formula for predicting IMC growth rates could help improve the strength of welds during the fabrication of lightweight materials in the automotive and aerospace industries.