When estimating the natural frequency of wind turbine structures using the single-degree-of-freedom modeling, usually part of the tower mass is concentrated to the tower top. However, the ratio of concentration is usually specified without considering that tapered towers are common in practice, which may cause certain errors. Additionally, the behavior of wind turbine structures may also be considerably influenced by the foundation flexibility and the substructure of offshore wind turbines. In this paper, a rigid-base single-degree-of-freedom structural model was proposed for wind turbines with tapered towers based on Rayleigh's method. Then, for structural modeling of monopile-supported offshore wind turbines, the concentrated spring model was introduced to simulate the flexibility of the monopile foundation as well as the transition piece, and inertial and flexural effects of the substructure were also included. Accordingly, the closed-form solution of the natural frequency of wind turbine structures was derived and was verified to be satisfactory compared to existing numerical and experimental studies. This simplified structural model was also shown to approximate the seismic response of wind turbine structures. It was further used to demonstrate the influence the foundation scour on the natural frequency of offshore wind turbine structures, exhibiting its applicability to scour monitoring. (C) 2020 Elsevier Ltd. All rights reserved.