The primary sigma factor ($sigma^{A}$) of Staphylococcus aureus, a potential drug target, was little investigated at the structural level. Using an N-terminal histidine-tagged $sigma^{A}$ (His-$sigma^{A}$), here we have demonstrated that it exits as a monomer in solution, possesses multiple domains, harbors primarily $alpha$-helix and efficiently binds to a S. aureus promoter DNA in the presence of core RNA polymerase. While both N- and C-terminal ends of His-$sigma^{A}$ are flexible in nature, two Trp residues in its DNA binding region are buried. Upon increasing the incubation temperature from 25$^{circ}$ to 40$^{circ}C$, $sim$60% of the input His-$sigma^{A}$ was cleaved by thermolysin. Aggregation of His-$sigma^{A}$ was also initiated rapidly at 45$^{circ}C$. From the equilibrium unfolding experiment, the Gibbs free energy of stabilization of His-$sigma^{A}$ was estimated to be +0.70 kcal $mol^{-1}$. The data together suggest that primary sigma factor of S. aureus is an unstable protein. Core RNA polymerase however stabilized $sigma^{A}$ appreciably.