Eight cases were reported in order to elucidate the important role of electron microscopy (EM) played in diagnosis of knotty tumors. The diagnosis of tumors made by light microscopy (LM) could be confirmed, corrected or eliminated with EM, and the types and histogenesis of tumors could be decided more accurately with EM than with LM. But EM has its inherent limitations, so it is emphasized that diagnosis made by EM must rest on solid basis of LM, and sometimes EM should be combined with other methods, such as histochemistry and immunohistochemistry.