Objective To explore the delineation between normal variations and developmental anomalies (malformation) of human cardiac conduction system (CCS). Methods The CCS of 886 cases (737 cases were noncardiac death,149 cases died of sudden cardiac death) were examined histologically using the method designed by the authors that tissues containing the SAN and AVN were cut along the long axes of the nodes in 1-2 block and the HB perpendicular to its long axis 2-4 blocks.The morphology and causes of death in CCS of two groups were also studied. Results 1.There were congenital variations in human CCS with respect to the sizes,position and shape;2.There were postnatal variations in CCS accompanying the aging process;3.It was revealed that there were developmental anomalies in CCS with sudden cardiac death,which included fetal typed atrioventricular node (AVN) in adults, complete displacement of the AVN into the central fibrous body,complete separation of the HB into more than 3 bundles and the displacement of bifurcation of the HB into the root of tricuspid valve.These changes should not be grouped into normal variations,since they are related or potentially related to sudden cardiac deaths.Conclusion The following changes should be included in normal variation or CCS:displacement of the bifurcating portion of the HB into the pars membranacea of the ventricular septum;left sided deviation of downward and leftward displacement of that portion;displacement of less than 1/2 of the AVN in to the central fibrous body;displacement of the myocardium into the HB or LBB.;