Hydrogen addition into biogas can improve the combustion performance. However, hydrogen addition will also pose higher explosion severity. In order to study the influence of hydrogen addition on the biogas explosion, the deflagration characteristics of premixed hydrogen/biogas/air mixtures with the unity equivalence ratio were evaluated in a 10 L transparent duct. The hydrogen fraction lambda(H2) in the blended fuel and the methane fraction in biogas theta(CH4) were varied 0.1-0.6 and 0.4-1.0, respectively. The results show that when the mixture was less reactive, the low flame velocity facilitated the flame front deformation, leading to the more salient tulip flames. The profiles of the reduced flame velocity V/ESL(i.e., laminar burning velocity S-L and expansion ratio E) could be divided into three patterns, depending on the combination of lambda(H2) and theta(CH4). As the reactivity of the mixture increased, the flame velocity profile would shift from the first pattern to the second pattern then to the third one. However, the V/ESL was linearly scaled with the flame position Z(tip )irrespective of the biogas composition before the flame touched the sidewall, indicating that the effect of flame instability and acoustic wave on flame propagation was insignificant. The explosion parameters (e.g., the maximum flame velocity V m and the maximum overpressure P-m) were more sensitive to the hydrogen fraction than to the methane fraction. In addition, hydrogen addition was more effective in augmenting the explosion parameters for biogas with low methane fraction. For a fixed methane fraction, the V-m and P-m showed the linearly dependence on the hydrogen fraction defined by Yu et al. (Yu and Law 1986). Moreover, the increasing hydrogen fraction would gradually moderate the effect of the methane fraction in biogas on the explosion parameters. There was a good linear nexus between the P-m and the V-m irrespective of the composition of the mixtures. The V-m was well scaled with the ES L or the maximum H radical [H]max in the reaction zone of laminar flame. Therefore, the (ESL) and [H](max )provide the good indicators to predict the explosion parameters of the hydrogen/biogas fuel. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.