In this study, the initial materials including aluminum nitrate nonahydrate and silicon powder were used to produce alumina–mullite composite through solution combustion synthesis (SCS). This paper aimed to investigate the influence of ammonium nitrate added as co-fuel to urea on the formation of alumina–mullite. For this purpose, ammonium nitrate/urea ratios of 0, 0.2, 0.4 and 0.5 were used for SCS. It was revealed through thermo-chemical calculations that increasing this ratio enhances the adiabatic temperature of combustion and facilitates the formation of crystalline mullite that is confirmed by structural analysis. XRD patterns showed that with increasing ammonium nitrate/urea ratio, the mullite peaks are intensified, and its trend is into the crystalline structure. Hence the mullite volume fraction increased from 4.12 to 30.52%. SEM studies showed that this ratio decreases a mean particles size from 264 to 133 nm.