FePt nanoparticles with a size of 4nm were prepared by the polyol process according to the reaction route described by Sun et al. The particles were annealed at 550 o C and 580 o C for 30min in N 2 atmosphere, developing coercivities of up to 8.8kOe. From conventional transmission electron microscopy the coherence lengths of the self-assembly were found to be as large as 10μm in the as-prepared state and about 1μm in the annealed state. The degree of sintering is zero for the 550 o C annealed samples and only a small amount for the 580 o C annealed samples. Ordering of the as-prepared fcc structure of the FePt nanoparticles into the L1 0 structure as a result of annealing is studied by high-resolution electron microscopy. In this investigation monodispersed nanoparticles are frequently found to undergo partial chemical ordering to the hard magnetic L1 0 phase without a change in size. Qualitative HREM observations about the amount of ordering of monodispersed nanoparticles, the low degree of sintering of the samples and large coherence length of the self-assembly together with the high coercivity developed upon annealing suggest the potential production of self-assembled ferromagnetic FePt arrays in future high-density magnetic data storage.