S-8 is the most stable compound of elemental sulfur in solid and liquid phases, at ambient pressure and below 400 K. Three crystalline phases of S-8 have been clearly identified in this range of thermodynamic parameters, although no calculation of its phase diagram has been performed yet. alpha- and gamma-S-8 are orientationally ordered crystals while beta-S-8 is measured as orientationally disordered. In this paper we analyze the phase diagram of S-8 crystals, as given by a simple and flexible molecule model, via a series of molecular dynamics (MD) simulations. The calculations are performed in the constant pressure-constant temperature ensemble, using an algorithm that is able to reproduce structural phase transitions.