Abstract: To study the aerodynamic stiffness of super high-rise buildings in the process of vortex-induced vibration (VIV), wind tunnel tests of multi-degree-of-freedom (MDOF) aero-elastic models were carried out to measure the vibration frequency of the system directly. The effects of structural damping, wind field category, mass density, reduced wind velocity (Vr), as well as VIV displacement on the VIV frequency were investigated systematically. It was found that the frequency drift phenomenon cannot be ignored when the building is very high and flexible. When Vr is less than 8, the drift magnitude of the frequency is typically positive. When Vr is close to the critical wind velocity of resonance, the frequency drift magnitude becomes negative and reaches a minimum at the critical wind velocity. When Vr is larger than 12, the frequency drift magnitude almost maintains a stable value that is slightly smaller than the fundamental frequency of the aero-elastic model. Due to the self-limiting and nonlinear characteristics of the VIV mechanism and the relationship between the STD of VIV displacement response and the aerodynamic stiffness, the aerodynamic stiffness and drift magnitude of the system frequency are highly sensitive to the aspect ratio, mass density, inherent damping of structure, as well as the roughness of wind field. Finally, the concept of nominal reduced wind speed and real reduced wind speed were proposed. An empirical formula to assess aerodynamic stiffness was established.