A ferroelectric liquid crystal showing a de Vries phase is studied by the mean of dielectric and electro-optical measurements. Due to a strong electro-clinic effect close to the smectic A*-smectic C* phase transition, a non linear electrical response is observed. The non linear dielectric characterization of this compound and the confrontation with a theoretical model allow us to extract the Landau's parameters of this material. The results are discussed on the basis of other data obtained by different measurement techniques. As an example, we confirm the second order smectic A*-smectic C* phase transition.