The flow-field of a fuel/air mixing system with an axisymmetric lobed mixer was numerically investigated. Large-scale streamwise vortices are formed immediately downstream of the mixer trailing edge, stretched further downstream, and finally broken into fragments where more intense mixing occurs. Both numerical and experimental results indicate that the length required for streamwise vortices breakdown in the confined flow-field of an axisymmetric lobed mixer is much shorter than that in the case of planar lobed mixers subject to parallel freestreams. For the conditions studied, the streamwise vortices start to breakdown at three wavelengths downstream of the mixer trailing edge. Large-scale streamwise vortices are formed immediately downstream of the mixer trailing edge, stretched further downstream, and finally broken into debris where more intense mixing occurs. Both numerical and experimental results indicate that the length required for streamwise vortices breakdown in the confined flow-field of an axisymmetric lobed mixer is much shorter than that in the case of planar lobed mixers subject to parallel freestreams. For the conditions studied, the streamwise vortices start to breakdown at three wavelengths downstream of the mixer trailing edge.