CoPt nanoparticles were prepared by simultaneous thermally reducing Co(CH3COO)2 and Pt(acac)2 in oleylamine with a small quantity of oleic acid. The composition of the particles was controlled by changing the amount of the reactants. Transmission electronic microscopy reveals that Co 48 Pt 52 particles with an average diameter of 8.4 nm are steadily dispersed in octane in the presence of oleylamine and oleic acid. Selected area electron diffraction indicates that the as-prepared particles have a face center cubic structure. Magnetic properties of these particles measured by a vibrate sample magnetometer yield a coercivity of 1.194×104 A·m-1 and a saturation magnetization of 5.3 emu·g-1 . After annealing at 650 ℃ for 2 h under the flowing Ar, the coercivity increases to 9.552×104 A·m-1 according to partly phase transformation from face center cubic structure to face center tetragonal structure for the nanoparticles.