Both experimental and calculated laminar burning velocities of H-2/O-2/He mixtures were obtained, with equivalence ratios of 0.6-4.0, initial pressures of 0.1 MPa-0.5 MPa, initial temperature of 373 K, and dilution ratio of 7.0. Laminar burning velocities changed non monotonically with the increasing initial pressures at equivalence ratios of 1.0-3.0. The decrease of overall reaction orders can explain the non-monotonic relationship between the laminar burning velocities and initial pressures. Consumption and production of both H and HO2 radicals were also obtained to explain the decrease of overall reaction order. The competition of H and HO2 radical between elemental reactions were also discussed. The three body reaction R15 (H + O-2(+M) = H02(+M)) gained more H radical in the competition with R15 (H + O-2 = 0 + OH), producing more HO2 radical. Through the reaction pathway analysis, the restraint in production of both OH and H leaded to a reducing radical pool. The poorer reaction pool would restrain the overall reaction and lead to the reduction of overall reaction order and the non-monotonic behavior of the laminar burning velocity. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.