To investigate self-acceleration propagation characteristics of a laminar premixed flame, an experimental study of H-2/CO/air mixtures with various hydrogen fractions and equivalence ratios was conducted. The acceleration exponent and fractal excess were defined to quantitatively investigated flame self-acceleration in the transition and saturation stages. Also, the influence of flame inherent instabilities on the acceleration exponent in the transition stage were investigated. The results indicate that with an increase in the hydrogen fraction, the first and second critical radius decreased, the proportion of the transition (saturation) stage in the whole flame propagation process decreased (increased), and the acceleration exponent and fractal excess of the transition and saturation stages increased. Because of the limits of flame radius and different degrees of pulsation in the saturation stage, the acceleration exponent and fractal excess at the saturation stage measured do not show obvious regularity; the values are less than 1.5 and 0.33, respectively. When the hydrogen fraction in syngas is changed, the acceleration exponent in the transition stage showed a nonlinear decreasing trend with an increase in the effective Le number. The hydrodynamic instability usually increased with a decrease in flame thickness, and the acceleration exponent in the transition stage increased. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.