Microstructural evolution of the semi-solid Al-4Cu-Mg alloy prepared by SIMA at different deformation temperatures and deformation degree as well as strain rates during isothermal compression process was investigated. Microstructure of the semi-solid alloy under high temperature deformation condition was observed also. The results showed that the average grain size increased by 2%～9% with deformation temperature increase from 540 ℃ to 560 ℃ and increase by 3%～16% from 560 ℃ to 580 ℃ while decreased with deformation degree increase. The average grain size in large deformation area was smaller by 4%～15% than in small deformation area in same specimen. Average grain size increased and grain became round with strain rates reduction.