This paper investigates the thermoelectric characteristics of cross-flow planar type solid oxide fuel cell (SOFC) with natural gas as fuel by using a three-dimensional numerical model. The results reveal that temperature and reactant concentration increase gradually along the direction of fuel gas flow, and the reactant concentration increases in the first and subsequently decreases. In addition, the lower the temperature, the higher ideal electromotive force is as well as the less actual output electromotive force. The hydrogen concentration is positively correlated with the current density and the ideal electromotive force. However, increasing the mass flow continuously beyond the reasonable range can decrease the current and electrochemical reaction intensity. Variation in wall thickness was also simulated and found that increasing the thickness would result in higher intensity of electrochemical reaction and increased current density but at the cost of low efficiency in SOFC. Thus an optimal design can make a balance between fuel utilization and output power of SOFC.