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Unsteady numerical study has been conducted on combustion dynamics of a lean-premixed gas turbine with a swirl injector. A three-dimensional computation method utilizing large-eddy-simulation (LES) technique with finite rate chemical reaction was applied with the message passing interface (MPI) parallel architecture. The unsteady turbulent flame dynamics are carefully simulated so that the flow motion can be characterized in detail, showing fairly comparable results with the experimental data. It was observed that some fuel lumps escape from the primary combustion zone, and move downstream and consequently produce hot spots and large vortical structures in the azimuthal direction. The correlation between pressure oscillation and unsteady heat release is examined by the spatial Rayleigh parameter. In addition, it is shown that the complicate heat-release-structure can be precisely regenerated by means of modal analysis using proper orthogonal decomposition (POD).