The microstructure evolution of the Alnico8 alloy thermomagnetically treated at the temperatures of 800, 850, 865 degreesC was investigated by transmission electron microscope. The microstructure in both as-homogenized and thermomagnetically treated specimens consists of the rod-like Fe-Co rich phase and Ni-Al rich phase. During thermomagnetical treatment, the Fe-Co rich phase was coarsened to a certain size firstly, then split into smaller ones, which resulted in a more homogenous and finer microstructure. The result also indicates that the temperature of thermomagnetical treatment has great effect on the splitting process of Fe-Co rich phase. Based on the point of view of energy, the splitting process of Fe-Co rich phase was analyzed. (C) 2002 Elsevier Science B.V. All rights reserved.