There is evidently a difference between the dynamic and static Faraday effects in magnetooptical medium.The real part θ′t of the alternating Faraday rotation θt all is less than the real part θ′s of the static Faraday rotation θs,and the imaginary part θ″t all is bigger than the imaginary part θ″s.It is shown by the experimental result of Faraday effect in (BiTm)3(FeGa)5O12 films that the frequency dependence of θt has a direct bearing on the mechanism of magnetic lag in the high-frequency region of ω>104Hz,and is mainly depend on the magnetic hysteresis in the low-frequency region of ω≤103Hz.Even if ω→0,θt does not equal θs as a result of the magnetic hysteresis loss in the medium.It is also indicated by the theoretical analysis that it is impossible to realize completely the on off state of the switch for a modulator utilizing the magnetooptical medium with magnetic losses.