Contribution contains an application of acoustic emission method during instability and growth of fatigue crack in reactor pressure vessel specimen loaded by means of bending. Material of specimen was typical material of pressure water reactor vessel covered by austenitic cladding as a protection layer against corrosion. Specimen had shape of prism with defect in the middle where fatigue crack was created very close to austenitic cladding. Loading of specimen was done in special loading machine with defined speed of loading resp. increased force. Investigation of instability and propagation of inertial crack was observed by acoustic emission technique, potential drop method, by using of straingauges and by high-speed camera. Acoustic emission sensors were located from both sides of specimen. By this experimental technique was possible to observe "pop-in" of crack during loading. Different types of specimen with different mode of aging were investigated. Results of acoustic emission activity are described graphical curves in relation to individual stages of loading.