Abstract: Transonic control surface buzz is an obstacle in the designing and using processes of modern aircraft, which can cause damages or serious deformation of the control surface. The limitation of previous investigations on transonic buzz is that they failed to predict the instability boundaries, which are key parameters to aircraft designers. In this paper, towards the typical buzz cases (A/B/C types), the inducing flow and structure conditions as well as the physical meaning of these parameters are studied by using the ROM-based aeroelastic analysis model and CFD/CSD time domain simulation method. Relevant results show that these buzzes are all the result of the instability on the structural branch owing to the coupling effect between the least-stable fluid mode and the structural mode. The eigenvalue analysis further reveals that the occurrence of transonic buzz requires the followings:(i) the stability margin of the flow is low enough (often near transonic buffet onset and offset conditions); (ii) the structural frequency is between those of zero-pole points of the open-loop Bode graph. This study helps to understand the physics and to propose new suppression methods for transonic buzz.