This paper explicates the characteristics of high-order multi-mode combustion instability generated in a model GE7EA combustor that has a partially premixed flame, which can prohibit flashback when firing fast-burning hydrogen-containing fuels. Five test cases varying both the fuel composition and heat input were selected, which generates a high multi-mode without a fundamental mode. Multi-mode instability phenomena were observed by 11 dynamic pressure sensors in both the time and frequency domains and by OH planar laser induced fluorescence (PLIF) images, which were time-synchronized with a dynamic pressure signal. A distortion in pressure waves, which resulted from the super-position of several harmonic modes of combustion instability, was observed, whereas distortion in heat release was not clearly observed. PLIF images and their statistical analysis showed scientific clues that proved the dependency of the flame location and movement on the combustion instability. The driving source of the combustion instability of the longitudinal mode was not only the longitudinal variations but also the radial variations in the center of intensity (COI) of the flame images so the Pythagorean addition of the interquartile ranges of both longitudinal and radial directions was suggested as a good indicator of the size of the longitudinal mode's combustion instability. The time series trajectory of the COI also provided meaningful information on the combustion instability mechanism. A linear and longer COI trajectory was likely to generate more intense instability because the one-way movement of the flame can be more effective in driving instability by transferring consistent force in the same direction. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.