Solid oxide fuel cells (SOFCs) are expected to be the most efficient and versatile system for chemical to electrical energy conversion. One of advantages of SOFC is the possibility of direct utilization of various types of hydrocarbon fuels. It is however well known that fuel impurities in practical SOFC fuels such as sulfur compounds could cause SOFC poisoning or degradation. Therefore, understanding the effect of impurities on an atomic scale is essential to develop impurity-tolerant SOFCs with long-term durability. Here, we have studied the poisoning and degradation mechanisms of nickel anode by typical impurities, i.e. hydrogen sulfide, hydrogen chloride, etc., using density functional theory method.