Nanostructured TbMn6Sn6 powders have been obtained by mechanical milling cast alloys and subsequent annealing at different temperatures. Powders were synthesized by milling for 1, 3, 5, and 8 h, and annealed from 573 to 773 K for 20 min. For the TbMn6Sn6 powders milled for 3 h and annealed at 723 K for 20 min, the highest coercivity, about 12 kOe is achieved at room temperature. The powder’s coercivities increase with decreasing temperature. The average grain size of the optimum powders is about 14 nm according to the x-ray diffraction patterns by using the Sherrer’s formula. The smooth and nearly square demagnetization curve suggests a very fine and uniform grain size, and the enhanced Mr/Ms (∼0.6) ratio indicates the existence of intergrain exchange interactions among the TbMn6Sn6 grains. The observed remarkable improvement of magnetic hardening in powders is believed to arise from the single-domain size in the powders. © 2002 American Institute of Physics.