By using free standing film of ferroelectric liquid crystals, we can realize a system with finite size along helical axis. The dynamics of free-standing film of FLCs has been studied by electrooptical measurement under weak sinusoidal electric field with do bias field not to occur the switching of directors. We have measured the electrooptical spectra by varying the thickness of film L and the ratio of L to helical pitch P by changing temperature. The obtained spectra make good agreement with those predicted theoretically by Urbank and Zeks. In most cases, two relaxation modes are observed in the spectra. The relaxation frequency of the faster mode slightly varies around 1 kHz and that of the slower mode varies largely with the ratio L/P.