We propose a Landau model to describe the smectic C-isotropic phase transition. A general Landau theory for the coupled orientational and translational order parameters and including the tilt angle is developed. The conditions for the smectic C-isotropic phase transition and the stability conditions of the smectic C phase are calculated. On the basis of this model it is argued that the smectic C-isotropic phase transition is always first order. We present a detailed analysis of the question under which conditions a direct smectic C-isotropic phase transition prevails in comparison to smectic A-isotropic and nematic-isotropic transitions. The theoretical results are found to be in qualitative agreement with all published experimental results.