In recent years the acoustic emission technique has been increasingly applied for detecting flaw s and cracks in materials either under testing or in service. It is important that what is the origin of AE signals detected by AE transducers. The purpose of this paper is to detect AE source on ductile material such as pressure vessel steel, A533B. As the results of experimentation, the increase of AE events number is proportional to the increase of the volume of plain strain zone. The origin of AE signals is fracture of MnS inclusions which have small amplitude of AE waves.