Hyperthermophilic archaeon, Thermococcus onnurineus NA1 was reported to grow on formate producing hydrogen (H2). In this study, to sustain high H2 production rate and demonstrate the feasibility of mass production of H2, high cell density cultivation of T. onnurineus NA1 on sodium formate was employed under optimized conditions. From batch cultures, it was observed that the salinity of medium, significantly changed by the addition of formate salt and pH-adjusting agent, crucially affected cell growth and H2 production. With salinity carefully controlled between 3.7 and 4.6 %, 400 mM sodium formate was found to be an optimal initial concentration for maximizing cell growth-associated H2 production. Under optimal conditions, the repeated batch culture with cell recycling showed high cell density of OD600 of 1.7 in 3 and 30 L bioreactor, and the volumetric H2 production rate was enhanced up to 235.7 mmol L(-1) h(-1), which is one of the highest values reported to date.