In work deformation of threshing drum in a combine harvester was determined basing on a signal of dynamic imbalance. The measured values of imbalance and their angles using the IRD technique enabled identification of imbalance signal with circumferential positioning of a deformed beater slat. Thus, a basis was established to formulate the rules of diagnostic interference concerning the extent of beater slat deflection and the change of the threshing gap. Installed vibration transducers on drum thresher give a possibility to continuous control of drum deformation at a housing a deck computer of combine harvester.