Surfactant-assisted high energy ball milling technique is a new method of producing magnetic nanoparticles. In this study, permanent magnetic SmCo 5 nanoparticles and nanoflakes with high room-temperature coercivity values and narrow particle size distributions were produced by this technology and a subsequent size-selection process. The SmCo 5 nanoparticles with average particle sizes of 9.8 and 47.5 nm, exhibited room-temperature coercivity values of 6.8×10 4 and 7.3×10 5 A/m, respectively, while the SmCo 5 nanoflakes, with the mean particle size of about 1.4 μm and average thickness of 75 nm, showed excellent permanent magnetic properties with an obvious c-axis crystal texture, a strong magnetic anisotropy and high coercivity values of 5.5×10 5 and 1.6×10 6 A/m in their easy-axis and hard-axis directions, respectively. The coercivity values of SmCo 5 nanoparticles and nanoflakes exhibited a significant particle size dependance effect.