Fusion bonding of cast iron and Al alloy is an effective way to improve the properties such as low inertia, high efficiency and corrosion resistance in machinery parts. In case of fusion bonding, intermetallic compound layers are formed at the interface between cast iron and Al alloy interface. It is important to control the intermetallic compound layers for improving bonding strength. The formation behavior of intermetallic compound layer by heat treatment has been investigated. Heat treatment was performed at temperature from $600^{circ}C$ to $800^{circ}C$ with $100^{circ}C$ interval for an hour to investigate the phase transformation during heat treatment. Heat treated specimens were analyzed by using FE-SEM, EPMA and EDS. The EPMA/WDS results revealed that various phases were formed at the interface, which exhibited 4 distinct intermetallic compound layers such as ${ au}_6-Al_{4.5}FeSi$, ${ au}_2-Al_3FeSi$, ${ au}_{11}-Al_5Fe_2Si $and ${eta}-Al_5Fe_2$. Also, fine precipitation of ${ au}_1-Al_2Fe_3Si_3$ phase was formed between ${ au}_{11}$ and ${eta}$ layer. The phase fraction in intermetallic compound layer was changed by heat treatment temperature. At $600^{circ}C$, intermetallic compound layer of ${ au}_6$ phase was mainly formed with increasing heat treatment time. With increasing heat treatment temperature to $800^{circ}C$, however, ${ au}_2$ phase was mainly distributed in intermetallic compound layer. ${ au}_1$ phase was remarkably decreased with increasing heat treatment time and temperature.