A new method is proposed to jointly estimate the shape parameter of a glottal model and its time position in a voiced segment. We show that, the idea of phase flatness (or phase minimization) used in the most robust Glottal Closure Instant detection methods can be generalized to estimate the shape of the glottal model. In this paper we evaluate the proposed method using synthetic signals. The reliability related to fundamental frequency and noise is evaluated. The estimation of the glottal source is useful for voice analysis (ex. separation of glottal source and vocal-tract filter), voice transformation and synthesis.