This study presents a linear programming model for an optimal long-term electricity mix planning of an insular territory. A prospective exercise on electricity generation is conducted over a period of 15-35 years. A general methodology is presented to determine an optimal generation mix that minimize greenhouse gas (GHG) emission by taking into account the environmental challenges of the territory. This methodology is applied to the case of Reunion Island. The main constraints considered here are the evolution of electricity emission factor and to meet the future electricity demand. Due to the long time-frame that we consider, a normative electricity mix scenario is presented, which aimed to reach the target by adjustments of the current electricity power system. The obtained results refer two scenarios: preserving and transforming structure. Referring to results, it is feasible to achieve environmental policy established by the energy transition act.