To investigate the effect of flame inherent instabilities on the flame structural characteristics of H-2/CO/air mixtures, experiments were conducted in a constant combustion bomb at various hydrogen fraction and equivalence ratio at room pressure and temperature. Wavelet transform was adopted to decompose the disturbances with different scales in the flame front. To study quantitatively the flame geometric structure characteristics, the maximum and minimum fluctuation radius growth rate, fluctuation range growth rate and energy of decomposition component were defined. The results indicated that with the flame development the disturbances at different scales all get greatly promotion. The higher the scale of the decomposition, the greater the amplitude and the energy of the disturbances. The disturbances at lower decomposition scales are the dominant component of the flame geometric structure characteristics; the ones at higher decomposition scales have a significance influence on the flame local geometric structure characteristics. When the flame development approached to a certain degree, the fluctuation range growth rate increased with the flame development. With the decrease of the equivalence ratio or the increase of hydrogen fraction, the thermal-diffusive instability enhances, leading to an increase of the fluctuation range growth rate and the relative energy of the approximate component at decomposition scale nine. (C)2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.