To find the optimum process parameters in the rheocasting process, we investigated the morphology and size of primary solid phase for various pouring temperatures and a different holding time in the semi-solid region. The semi-solid slurry of Al alloy was produced in a precisely controlled slurry maker with a multiplex type slurry cup. The microstructure of the interrupt-quenched semi-solid slurry during isothermal holding was observed, and the morphology of primary solid phase was examined. The morphology of the primary solid phase was coarsened with increasing pouring temperature and holding time, and the initial morphology of primary solid phase was not largely changed as the isothermal holding time increased up to 10 min. Therefore, the morphology of the primary solid phase in semi-solid slurry was considered to affect the initial microstructure that appeared at the solidification start time.