In this study, $BaTiO_3$ thin films were grown by RF-magnetron sputtering, and the effects of a post-annealing process on the structural characteristics of the $BaTiO_3$ thin films were investigated. For the crystallization of the grown thin films, post-annealing was carried out in air at an annealing temperature that varied from $500-1000^{circ}C$. XRD results showed that the highest crystal quality was obtained from the samples annealed at $600-700^{circ}C$. From the SEM analysis, no crystal grains were observed after annealing at temperatures ranging from 500 to $600^{circ}C$; and 80 nm grains were obtained at $700^{circ}C$. The surface roughness of the $BaTiO_3$ thin films from AFM measurements and the crystal quality from Raman analysis also showed that the optimum annealing temperature was $700^{circ}C$. XPS results demonstrated that the binding energy of each element of the thin-film-type $BaTiO_3$ in this study shifted with the annealing temperature. Additionally, a Ti-rich phenomenon was observed for samples annealed at $1000^{circ}C$. Depth-profiling analysis through a GDS (glow discharge spectrometer) showed that a stoichiometric composition could be obtained when the annealing temperature was in the range of 500 to $700^{circ}C$. All of the results obtained in this study clearly demonstrate that an annealing temperature of $700^{circ}C$ results in optimal structural properties of $BaTiO_3$ thin films in terms of their crystal quality, surface roughness, and composition.