The morphological evolution and size of primary solid phase during the isothermal holding at a different holding temperature in semi-solid region were investigated to examine the globularization and coarsening behavior of primary solid phase in semi-solid slurry. The semi-solid slurry of Al alloy was produced in a precisely controlled slurry maker with a slurry cup. The size and roundness of primary solid phase in the quenched slurry after a given isothermal holding time at a different holding temperature in semi-solid region was observed and measured. The morphology of the primary solid phase was considered to be largely affected on the initial microstructure of the semi-solid slurry produced during continuous cooling stage. The size of the primary solid phase was decreased with increasing holding temperature, and the primary solid phase was coarsened with increasing holding time. The roundness degree of the primary solid phase of the quenched slurry after holding at 580 degrees C was approximately 1.6 at a holding time of 1min. and reduced to 1.2 during the isothermal holding for 60min.