Among many ferromagnetic materials, CoPt alloy nanoparticles with high coercivity and small grain size are one of the potential candidates having its application in high-density storage media manufacturing. CoPt alloy nanoparticles of different compositions have been prepared in TX-100 micellar medium and capped with sodium oleate to prevent the agglomeration during annealing at high temperature. Their structural and magnetic properties were investigated before and after heat treatment. The particles with similar to 50:50 compositions of Co and Pt gives L1(0) phase after annealing at 900 degrees C for 1 h and show a high coercivity (similar to 4.4 kOe at room temperature) and remanence due to its highly anisotropic L1(0) structure. (C) 2009 Elsevier Inc. All rights reserved.