Based on TGZM (temperature gradient zone melting) effect ,experiments on Co-87.9wt%Sb alloy consisting of melting followed by thermal stabilization ranging from 20min,2h,4h and quenching were carried out in a directional solidification apparatus, after which the solidified phases and the composition distribution were researched. The results showed that, under the temperature gradient, there exists a mushy zone between the complete liquid zone and the non-melted solid zone. Along the direction of the temperature gradient, the mushy zone of Co-87.9wt%Sb alloy is divided into three parts including (CoSb3+L),(CoSb2+L) and (CoSb+L). Due to TGZM effect, the volume fraction of the liquid phase in the mushy zone gradually decreases and the interface between the mushy zone and the complete liquid zone moves towards the low-temperature zone as thermal stabilization time increases. According to the composition distribution of the mushy zone, after thermal stabilization, the solute concentration obviously deviates from the initial concentration of Co-87.9wt%Sb alloy which is smaller than that of the complete liquid zone. Theoretical analysis and computation explain the above experimental observations. After 4h thermal stabilization, the volume fraction of CoSb3 in (CoSb3+L) of the mushy zone increases significantly up to 98.8%, which shows that it is feasible to prepare CoSb3 material via TGZM effect.