The results on atomic ordering and structures of hard magnetic L1 0 FePd nanoparticles studied by modern electron microscopy techniques, including NBD, CS-corrected HRTEM, HAADF-STEM, & 3D electron tomography are discussed. The key issue throughout the study is the atomic ordering in the alloy nanoparticles, in view of excellent hard magnetic properties due to the high magnetocrystalline anisotropy energy (MAE), which is dependent on the long-range order (LRO). The kinetics of atomic ordering plays a crucial role, that is, annealing below a reduced order-disorder transformation temperature is essential in obtaining the L10 ordered phase in small nanoparticles. A novel technique to measure the degree of order in individual particles using NBD and diffraction intensity calculations was proposed, & quantitatively proved the size-dependent degradation of long-range atomic order in nanoparticles. Imaging using CS-corrected HRTEM revealed the atomic structures of the alloy nanoparticles 2-10 nm in diameter.