An experimental study was conducted under various operating conditions with a model gas turbine combustor to examine the relation between combustion instability and fuel composition. Mode analysis was conducted in unstable conditions based on measured dynamic data, and the flame structure was visualized by OH-chemiluminescence and the phase-resolved OH-PLIF measurement technique. Heat input and inlet air temperature variation experiments using various fuel compositions showed that combustion instability occurs under certain conditions and the instability frequency varies based on fuel composition. Fuel composition influences the combustion dynamic characteristics and three kinds of instability frequency ranges were found under the same geometry and heat input conditions. These instabilities are the 3^rd, 4^th, and 6^th harmonics of the fundamental longitudinal mode, and the composition of the fuel affects the occurrence of various modes. Dump plane temperature characteristics were also studied at unstable conditions and the reason for the temperature variation was found using flame visualization techniques.