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The structure and thermoacoustic instability (TI) of premixed CH 4 /H 2 /air swirling flames were experimentally investigated for a range of hydrogen fraction (η H2) up to 80% under different equivalence ratio (Φ) and swirl number (S) conditions. It is shown that the onset of TI is enhanced when increasing either η H2 , S , or Φ. The dominant frequency of TI increases dramatically with η H2. The higher dominant frequency in the hydrogen-enriched flames can be attributed to a shorter flame length which results in a reduced flame convection time. It is observed that the unstable flames are always accompanied by the appearance of outer recirculation zone (ORZ) flame. Therefore, the flame kernel residing in the ORZ can be an indicator of the occurrence of TI. The flame front of thermoacoustic unstable flames was observed to be more wrinkled, e.g., with larger mean absolute curvature (κ abs) and local flame surface area ratio (δ Σ max). Importantly, the phase-locked analysis shows that κ abs and δ Σ max can be modified at different oscillation phases, and their maximum and minimum values are simultaneously achieved at phase angles θ of about 0° and 180°, respectively. Variations of κ abs and δ Σ max are in phase with the heat release rate, indicating a strong correlation between the TI and flame structure modification; however, they show a phase lag of about 72° behind the pressure in this work. These results are vital when understanding and predicting the TI based on the flame structure, especially when adopting a visual detection method of the instability. [Display omitted] • Thermoacoustic instability of swirling lean premixed CH 4 /H 2 /air flames is studied. • The dominant frequency increases significantly with hydrogen fraction. • Appearance of the ORZ flame is highly related to the instability. • The flame front is more wrinkled in unstable flames. • Flame curvature and surface area oscillate in phase with heat release rate. [ABSTRACT FROM AUTHOR]