The adsorption of iodine atoms and molecules on graphene is studied in detail, using first-principles calculations that include non-local correlation effects by means of van der Waals density functional approach. Structural, energetic and electronic structure properties of these systems are reported. We demonstrate that graphene surface can be doped by atomic and molecular iodine. An upward shift of the Dirac point from the Fermi Level, with values of 0.45 eV and 0.08 eV are observed, for adsorbed atoms and adsorbed I2 respectively . It corresponds to graphene hole densities to be around 1.2 × 10^13 cm−2 to 3.9 × 10^11 cm−2. We also show that the iodine molecule does not dissociate in contact with pure graphene monolayer. Calculation of the surface free energy reveals that the orientation of the adsorbed iodine molecules crucially depends on its concentration and the system temperature. The corresponding phase diagram indicates that the in-plan orientation of molecules is more stable when the iodine con- centration decreases for temperature above approximately 200 K, when beyond 500 K, iodine molecules are completely desorbed.