Effects of the buoyancy induced by chemical heat release in a diffusion flame were investigated using direct numerical simulation (DNS). A parametric study by varying the Froude number of the flow was performed to examine the unsteady effects of combustion-induced buoyancy. Chemical reaction was governed by a one-step Arrhenius kinetics. Simulations with and without external disturbance were performed, which revealed that the buoyancy instability is a Row instability different from the jet preferred mode of instability. By varying the Froude number, three types of vortical structures were observed in the Row field of the disturbed reactive plumes, including inner vortical structures due to the jet preferred mode of instability, outer vortical structures due to the instability associated with combustion-induced buoyancy, and the coexistence of inner and outer vortical structures in the Row field. The vorticity transport equation was analyzed to examine the mechanisms leading to the formation of large vortical structures in buoyant reactive plumes. The interaction between the combustion-induced radial density gradients and the gravity played a predominant role in the vorticity generation and transport precesses in buoyant diffusion flames. The density inhomogeneity in the radial direction subjected to gravity was essential to the development of combustion-induced buoyant flow instability,and it led to the formation of counterrotating vortices in buoyant reactive plumes.