An experimental study was conducted to identify a mechanism of combustion instability due to flame-vortex interactions and to characterize the effects of the interaction on self-excited unstable combustion in a swirl-stabilized lean-premixed gas turbine combustor. A variable-length combustor was designed to determine the preferred instability frequency at a given operating condition, and natural gas fuel was injected far upstream of the choked inlet to the mixing section to eliminate the possibility of equivalence ratio fluctuations. Experiments were performed over a broad range of operating conditions to simultaneously measure dynamic pressures in the combustor and mixing section and the overall rate of heat release. Acoustic velocity at the combustor inlet was obtained by the two-microphone technique, and two-dimensional CH* chemiluminescence flame images were taken to visualize stable and unstable flame structures and vortex dynamics during unstable combustion. The normalized heat release response versus the velocity fluctuation of the self-excited flames showed two regimes of response characteristics-linear and non-linear-which was analogous to forced flame responses. The phase synchronized CH* chemiluminescence images revealed flame-vortex interaction regardless of the instability modes or operating conditions for strong instability. Detailed modeshapes of the characteristic instability modes were investigated by thermoacoustic modeling, and verified with the extensive dynamic pressure measurements along the wall of the mixing section. Two distinct instability characteristics were observed as 1L mode at around 220 Hz and 2L mode near 350 Hz. It was deduced that the 1L mode instability was caused by the flame's active response to a perturbation at the frequency near 220 Hz, and the 2L mode instability was amplified by the system's gain related to the acoustics of the mixing section.