Flow difference between reacting and non-reacting flow in a swirl stabilized annular combustor is investigated using 3D Large Eddy Simulation with level-set flamelet turbulent combustion model. The combustor of concern is the LM6000, lean premixed dry low-NOx annular combustor, developed by GEAE. Boundary conditions are based on experimental data. Strong vortex breakdown in main stream, vortex ring proceeding downstream, and the turbulent structure periodically oscillating have been observed. Heat release as a result of combustion drastically increases the dilatation of density in primary combustion zone so that the main swirl stream behind of a swirl cup stretched further downstream along shear layer than non-reacting flow. The oval shape of core flow in cross-section to flow direction, which clearly observed in non-reacting case, tends to be circle shape, and small vorticities in wide range of non-reacting flow disappears, but the size of iso-vorticity is stretched to flow direction in reacting flow.