The influences of hydrogen addition on laminar premixed fuel-rich formaldehyde flames at the atmospheric pressure are studied by the detailed kinetic analysis. The chemical effects of hydrogen addition on flame structures, mole fractions of major species and free radicals are distinguished clearly from the dilution and thermal effects. The results show that the H-2 chemical effects can promote the oxidation of formaldehyde and make the mole fraction profile of formaldehyde move toward the upstream side. The increase of H radical concentration is mainly due to the chemical effects of hydrogen addition. The reaction OH + H-2 = H + H2O plays a major role in the productions of H and OH radicals. The productions of O, HCO and HO2 radicals are enhanced by the H-2 chemical effects. However, the hydrogen dilution and thermal effects on decreasing these radicals' concentrations are more dominant. The mole fractions of CO and CO2 decrease with H-2 addition. The H-2 chemical effects facilitate the formation of CO, but suppress the production of CO2. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.