The 30-cell (10 x 10 cm(2)) LSCF-GDC (La0.6Sr0.4Co0.2Fe0.8O3-delta - Ce0.9Gd0.1O1.95) oxygen electrode supported planar solid oxide electrolyzer cell (SOEC) stack module is manufactured and tested at different operation conditions (operating temperature, steam content, gas flow rate et al.) in steam electrolysis mode for hydrogen production. The i-v curves of each electrolysis are recorded and fitted according to the electrochemical models, the influences of operation conditions on the polarization loss of high temperature water electrolysis are analyzed, and the electrolysis efficiencies are calculated. High operating temperature gives the reduction of Ohmic and activation polarization, and high steam content results in significantly lowered activation polarization. The flow rates of the gas onto the hydrogen electrode and oxygen electrode have great impact on concentration and activation polarization. The electrolysis efficiency can reach the maximal value of 80.8% (electrolysis voltage 1.273 V) when the system operating at 800 degrees C, with 6 L min(-1) 90% humidified hydrogen mixed gas, and the rate of resultant hydrogen is 4.57 L min(-1). (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.