A reduction of bismuth oxide in hydrogen atmosphere was investigated. The reaction was performed with a material in various structural forms: powder: with micrometric grains, powder with nanometric grains and powder pressed into pellets. The process was performed in both isothermal and non-isothermal conditions. An activation energy of the reaction calculated with Friedman method was found to be about 85 kJ/mol for the reduction of both micrometric powder and pellets. A model fitting analysis based on Coats-Redfern method suggests, that the reaction is limited by a diffusion of gaseous reactants, what is consistent with a structural analysis. An effect of liquid bismuth evaporation was noticed at the beginning of the process. This phenomenon was much stronger in the case of reduction of nanometric powder. The activation energy was estimated to be about 30-50 kJ/mol. The reaction could have been performed at lower temperature.