Intumescent coating is an insulating system designed to decrease heat transfer from afire to the structure that is being protected so that the structure can maintain its integrity within one to three hours for evacuation process. This study focuses on the bonding mechanism of expandable graphite based intumescent coating on steel substrate as poor bonding of intumescent flame retardant coating with steel substrate will lead to failure of the intumescent coating after fire test. The Intumescent coating composition contains expandable graphite (EG) as a carbon source, ammonium polyphosphate (APP) as acid source, melamine (MEL) as blowing agent and zinc borate as additive, bisphenol A as epoxy resin and polyamide amine as hardener. The steel is mill blasted to improve adhesion and corrosion resistance. The physical properties and morphology are investigated using FESEM and SEM after fire test. Different formulation was prepared with different weight percentage of the ingredients to determine better bonding between intumescent coating and steel substrate. Graphite size 300 pm is used as it gives better expansion thus promotes adhesion. The effect of bonding strength of expandable graphite based intumescent coating with respect to its physical properties on steel substrate is investigated.