The colloidal synthesis and magnetic properties of MnPt3 nanocrystals are reported. The nanocrystal size depended on the Mn reactant used, but the Mn:Pt stoichiometry was always 1:3. As synthesized, the nanocrystals are compositionally disordered with the face-centered cubic (fcc) A1 phase. Annealing at 580 C changed the MnPt3 crystal structure to the compositionally ordered Ll(2) phase (AuCu3 structure) with higher magnetocrystalline anisotropy. Magnetization measurements showed that the A1 nanocrystals are paramagnetic and the Ll(2) MnPt3 nanocrystals are superparamagnetic.