This study presents the fire resistance results of incorporating nano-sized alumina in boron nitride reinforced epoxy-based intumescent fire-retardant coating. Structural steel substrates were coated with the developed coating and exposed at 800 °C for 2 h for furnace fire test to examine the char expansion and morphology. The fire-resistance of the coating was tested by heating the coated substrates using a Bunsen burner for 1 h. The coated substrates were tested for water absorption followed by a fire-resistance test. The furnace fire test showed that boron nitride-alumina (BNA-1) reinforced with 0.1 wt of nano-sized alumina resulted in 45 higher char compared to boron nitride-alumina reinforced with 0.4 wt (BNA-4) and boron nitride-alumina reinforced with 0.5 wt (BNA-5). The increased amount of alumina improved the mass of the char. The lowest substrate temperature of 137 °C was recorded by BNA-5 with a maximum char mass of 43.89 wt. This was considered due to formation of aluminum oxide and aluminum phosphate on the char surface, reduced char pore size, and increased amount of alumina in the gas emission. X-ray Diffraction (XRD) analysis found boron phosphate, boron nitride, and phosphoric nitride compounds were reliable in protecting the substrate at high temperature. The functional groups of char compounds were observed by FTIR. FESEM micrographs showed the presence of nano-sized alumina. XPS results also suggest that nano-alumina may react with boron nitride and ammonium polyphosphate additives, producing aluminium nitride, boron oxide, and aluminium phosphate that are beneficial to the intumescent process and fire-resistant performance. Pyrolysis GC-MS results showed reduced emission of gaseous product in BNA-5. This concluded that the addition of 0.5 wt alumina in BNA improved char morphology and increased char mass. No changes in the microstructure and hardness of the substrate were observed after the fire test. The fire resistance of the water-immersed samples showed a small increase in substrate temperature which was within the acceptable range for structural safety. © 2020 Elsevier B.V.