The FePt nano-particle structure has been considered to be a candidate for the next generation of high-density recording media due to its large magnetocrystalline anisotropy. The large magnetocrystalline anisotropy originates in L10 ordered structure of the FePt phase. However, it has been reported in recent investigation that disordering takes place when the FePt particle size is smaller than a critical size for ordering. In this study, the ordering of FePt nano-particle is modeled using the phase-field method, and the size dependence on the ordering of FePt nano-particle is analyzed based on this model. As a result, the disordering in the FePt particle proceeded rapidly when the particle size is less than the critical size for ordering. This phenomenon is due to an increase of the volume fraction of the disordered region at the vicinity of particle surface, when the particle size decreases.