CO2 fixation by a hydrogen -oxidizing bacterium, Cupriavidus necator, was evaluated in a packed bed bioreactor under a constant flow rate of gas mixtures (H-2, O-2, CO2). The overall energy efficiency depends on the efficiencies of CO2 fixation into carbohydrate and the reduced carbon into biomass and bioproducts, respectively. The efficiencies varied with the limiting gas substrate. Under O-2 limitation, the efficiency (20-30%) of CO2 fixation increased with time and was higher than the overall efficiency (12-18%). Under H-2 limitation, the efficiency of CO2 fixation declined with time while the biomass yield was quite similar to that under O-2 limitation. A cellular metabolic model was suggested for the lithoautotrophic growth of C. necator, including CO2 fixation into carbohydrate followed by the main metabolic pathway of reduced carbon. Under CO2 limitation, most H-2 energy was wasted, resulting in a very low biomass yield. Under a dual limitation of O-2 and nitrogen, biosynthesis of poly(3-hydroxybutyrate) was triggered, and the energy efficiency or yield of biopolyester was lower than those of microbial cell mass. Compared with a green microalga Neochloris oleoabundans that produces lipid under nutrient limitation, C. necator exhibited a much higher (3-6 times) energy efficiency in producing biomass and bioproducts from CO2.