Abstract: Different faults of oil-immersed power transformers were classified with three-layer structure of 'parts-fault causes-temperature rising phenomeno' and the fault causes were summarized, impacts of the different faults on outer surface temperature of transformer were briefly analyzed. The results show that fault will lead to outer surface temperature rising which including iron core and clamping pieces, winding and leading wires, tapping switch, insulation system, tank and base of the transformer; the faults of cooling system inevitably lead to outer surface temperature rising; part of bushing failures of outlet and inlet line will cause outer surface temperature rising of the transformer; most of failures of protection and measurement device will cause temperature rising of the transformer. Thus the preliminary relations between faults and its outer surface temperatures of the oil-immersed power transformer were obtained, namely most faults of the transformer will lead to outer surface temperature rising.