Abstract: Frequency lock-in resonance in ice-induced vibration (IIV) threatens severely the safety of the structures and worsens the working environment for operators. Its underlying mechanism is unclear yet. This paper presents a theoretical study on the mechanism of the frequency lock-in resonance for compliant structures. The study is based on an intermittent ice-crushing type of IIV model developed previously by the author and co-worker (Huang and Liu, 2009). The frequency lock-in resonance is predicted over an ice velocity span. Then the parametric analysis is performed on IIV and frequency lock-in resonance for some influential factors, including structural damping and stiffness, ice stiffness, ice-crushing zone length, ductile-brittle transitional ice-velocity and randomness in the ice-crushing strength and ice-crushing zone length. From these theoretical studies, the mechanism of the frequency lock-in resonance is investigated. It is shown that although both the predominant ice and structural response frequencies are locked to the structural natural frequency when frequency lock-in resonance takes place, the time history profiles of ice force and structural response and their frequency spectra are different corresponding to the different ice velocity. Not only the conventional mono-frequency resonance with the uniform amplitude but also the multi-frequency beat resonance with the periodically changing amplitudes are predicated. For the frequency lock-in resonance, structural and ice properties affect the length and location of the ice velocity span as well as the response amplitude, and the randomness and strain rate effect in ice-crushing are the two competing factors. It is unveiled that the strain rate effect of the ice-crushing strength is responsible for the frequency lock-in resonance, by frequency modulation and by promoting the uneven kinetic energy transfer between ice and structures, a positive feedback mechanism. The present novel mechanism is a coupled vibration that is essentially different from the conventional one, i.e., the negative damping in self vibration predicted from the continuous ice-crushing type of IIV models. The present result is instructive to the further systematic experimental study on the frequency loc-in resonance and to devising some effective techniques for the mitigation of intensive IIV.