Most of the researches about the wind turbines frequency regulation are based on the real-time simulation therefore fail to evaluate the frequency regulation capability quantitatively. To deal with this, this article derives the system frequency response model with wind turbines (WTs) participating in the frequency regulation under the deloaded and maximum power point tracking (MPPT) status. The frequency nadirs (FN) under the two status are calculated according to the system frequency response model and the WT frequency regulation capability can be evaluated by FN quantitatively.Under the deloaded working status, the FN can be obtained by directly taking the inverse Laplace transformation of the system frequency response model. However under MPPT status it is difficult to calculate the FN by directly taking the inverse Laplace transformation due to the high order of the frequency response model. Therefore an order reduction method by negelecting the non-dominant poles is proposed. The FN is calculated through the reduced model. Then the sensitivities of FN with respect to the frequency controller gain and the WT initial speed can be obtained. The sensitivities indicate that the FN is affected both by the controller gain and the WT initial speed. The FN can be improved by increasing the controller gain while the increase of the initial WT speed results in a lower FN. Finally, the influence on the FN of each factor is demonstrated by simulation.