Compositionally ordered, single domain, antiferromagnetic L1(2) FePt3 nanocrystals were synthesized by coating colloidally grown Pt-rich Fe-Pt nanocrystals (Fe0.27Pt0.73) with thermally stable SiO2 and annealing at 700 degrees C in forming gas (7% H-2 in N-2). Without the silica coating, the nanocrystals transform predominately into the L1(0) FePt phase due to interparticle diffusion of Fe and Pt atoms. Magnetization measurements of the L1(2) FePt3 nanocrystals revealed two antiferromagnetic transitions near the bulk Neel temperatures of 100 and 160 K. Combining L1(2) FePt3 nanocrystals with L1(0) FePt nanocrystals was found to produce the constriction in field-dependent magnetization loops that has been observed near zero applied field in ensemble measurements of single domain silica-coated L1(0) FePt nanocrystals [Lee, D. C.; et al. J. Phys. Chem. B 2006, 110, 11160].