Performance of solid oxide cells (SOCs) is investigated under both fuel cell and electrolyzer operations to understand their asymmetric behavior between the two operation modes. The current-voltage characteristics and the electrochemical impedance of an anode-supported cell are measured at the open circuit potential and also at the biased conditions. The partial pressure of the supplied gas and the load current are varied to evaluate their effects on the cell performance. The gas partial pressure of hydrogen and water vapor supplied to the hydrogen electrode are kept equivalent so that the performance can be compared under the same gas utilization between the fuel cell and electrolyzer modes when the same current density is applied. The polarization resistance obtained from the impedance measurement is separated into several internal resistance components by performing equivalent circuit fitting and the factors that are related with the asymmetric behavior of the SOCs are discussed. It is found that the asymmetry of the cell performance is caused by multiple factors, such as activation resistance in the oxygen electrode and the diffusion resistance in the hydrogen electrode.