In this research, diffusion bonding was carried out to produce transition joints between mild steel A36 (Fe A36) and aluminium Al 5083 (AA5083) with the presence of gallium (Ga) as an interlayer between the two faying surfaces. The microstructural development and interfacial growth of intermetallic compounds at the interface layer between Fe A36 and AA5083 after the diffusion bonding process were investigated. The joining was performed by clamping the two materials with a Ga interlayer and then heated in a furnace. The interlayer developed from this diffusion heating in air condition provides an average thickness of 30 μm. Characterization of intermetallic compounds was conducted using SEM-EDX and XRD. The results showed that SEM-EDX confirmed the occurrence of interdiffusion of elements from Fe A36 and AA5083 present at interlayer. XRD analysis reveals the formation of Fe3Al at the diffusion layer.