One of the most interesting properties of the EIT is that its linewidth can be extremely narrow in a Λ-type configuration where two lower levels are hyperfine ones within a ground state. In such a configuration, the EIT linewidth is mainly governed by the coherence relaxation rate between hyperfine levels which can be of several orders of magnitude smaller than that of the optical transition. However, As intensity of coupling field increases, the EIT linewidth can be broadened. In order to control power broadening of EIT, we present a quasi-Λ four-level system, and introduce an microwave field to drive a hyperfine transition.It is shown that the microwave field not only splits an EIT window into a doublet but also narrows the EIT window. Power broadening of EIT window can be suppressed by utilizing a microwave field.