High temperature steam electrolysis (HTSE) through solid oxide electrolysis cells (SOEC), a promising high-efficiency and zero-emission way to large-scale hydrogen production, has been received increasingly international interest. The hydrogen production efficiency of HTSE is more than 50%. In this paper, the electrochemical performance and microstructure change of single button cells operating in both fuel cell (SOFC) and electrolysis modes (SOEC) were studied at 850°C. Also, the degradation mechanisms of hydrogen electrodes were investigated. The results showed that OCV decreased from 0.944 V to 0.819 V when the steam content increased from 20% to 80%. The voltage began to increase rapidly at relatively higher current density for lower steam content because of steam starvation; however, steam starvation did not occur at higher steam content. The ASR data decreased from 1.68 to 0.645Ωcm2 with the increase of steam contents, while steam content had little effect on ASR data in SOFC mode. The polarization loss of the single cell was higher in electrolysis mode than that in fuel cell mode. The microstructure of the hydrogen electrode changed obviously after electrolysis process. Furthermore, the performance degraded at high steam partial pressure due to the oxidation of Ni grains at the interface of hydrogen electrode.