Abstract: Study of species characteristic and formation of microdefects in thermite welded joints in rails was described.The pattern of fracture by impact penetration test and tensile fatigue test of thermite welding with stereoscan was observed,and the component of defect with energy spectrum was analyed.The microdefects include micropores and inclusions.Microporosity in thermite weld joint consiste of blow hole and shrinkage cavities.The inclusions comprise agglomeration,pyramid,granule,accumulation,filament and mixed patterns according to morphological analysis and measurements.The inclusions have several causes:segregation with a high-content of sulfur or phosphorus;oxides with a high-content of oxygen and compound metals;and un-dissolved substances such as Fe or Si as a result of unsuitable processing.