Graphite response to irradiations has been widely studied in the past because of its importance for nuclear engineering. Although its behavior under irradiation has been widely investigated, the very details of the underlying mechanisms are still under debate and several scenarios are available. With molecular dynamics simulations using empirical potentials we investigate in this paper the microstructural evolution under irradiation of two types of graphite at different length scales graphite single crystal and polycristalline graphite mimicking the real nuclear graphite structure.Primary damage was investigated by displacement cascades. While mainly point defects only survive in graphite single crystal, the nanoporosity of polycristalline graphite modifies the nature of the irradiation damage.Dose effect was subsequently explored by means of Frenkel pair accumulations in graphite single crystal. Before amorphisation, we show that graphite single crystal follows a three stages evolution characterized by an increase of point defects a wrinkling and pinning of the graphene planes at small amorphous pockets and an amorphisation by percolation of the small amorphous pockets. Although each of these three stages have already been proposed, their chronology is evidenced in the present simulations