Island regions are not interconnected with the mainland and the isolation of these regions has brought challenges to the electric power systems. For instance, the power grids in island regions are more fragile to the rapid variance of the electricity consumption or production than the continental grid. Meanwhile, more intermittent renewable energy, such as wind and solar, has been connected to the power grid. In this work, we propose a long-term unit commitment (UC) model with simplified network (zone) constraints to apply in these areas. The method is made of four steps. The first step is to simulate short-term market outcomes by using an economic dispatch model with a detailed nodal network. The second step is to find the optimal zonal configuration. The third step is to decide the simplified network constraints for the long-term unit commitment. The fourth step is to apply the simplified network constraint in the long-term unit commitment problem. The method is applied for the case study of Martinique.