This paper concerns non linear dielectric spectroscopy in the SmC* phase of a ferroelectric liquid crystal. The experimental set-up and the procedure used to determine non linear permittivities are described. The results are discussed on the basis of the model proposed by Kimura et al. [Y. Kimura, S. Hara, R. Hayakawa, Phys. Rev. E 62, R5907 (2000); Y. Kimura, S. Hara, R. Hayakawa, Ferroelectrics 245, 61 (2000)]. A good agreement is obtained between the measurements and the theoretical predictions for $\epsilon^{*}_{3,3}$ and $\epsilon^{*}_{5,5}$ non linear permittivities. Some physical parameters of the material like the polarization, the rotational viscosity and the elastic term are determined. The polarization is found similar to this obtained with classical triangular wave method.