Effects of different solidified velocities on microstructure of directional solidification Al-17.5In monotectic alloy were investigated.The results show that regular indium fibres precipitated from the alloy by monotectic reaction are regularly arranged in the Al matrix at large gradients and low solidification velocities,while with increasing in solidification velocity,the indium fibres are gradually converted into pearl-string structure.The transformation of the structures is described,and a new criterion for the destabilization of regular indium fibrous growth in directionally solidified Al-17.5In monotectic alloys is derived.