In this paper, the frequency modulation effect of the probe field in a three-level Λ system is discussed. According to the classical equation of motion for the electron, the dielectric susceptibility of the medium is not affected by frequency modulation. Based on the quantum theory and the frequency modulation term expanded in terms of Bessel functions, the effects of the frequency modulation of the probe field on the system are studied, in which, when the modulation frequency of the probe field is large enough, the periodical population evolutions exist in three levels. Meanwhile, the stationary susceptibility of the probe field is given.