In voice analysis, the parameters estimation of a glottal model, an analytic description of the deterministic component of the glottal source, is a challenging question to assess voice quality in clinical use or to model voice production for speech transformation and synthesis using a priori constraints. In this paper, we first describe the Function of Phase-Distortion (FPD) which allows to characterize the shape of the periodic pulses of the glottal source independently of other features of the glottal source. Then, using the FPD, we de scribe two methods to estimate a shape parameter of the Liljencrants-Fant glottal model. By comparison with state of the art methods using Electro-Giotto-Graphic signals, we show that the one of these method outperform the compared methods.