Abstract: Semi-solid metal slurry was prepared by introducing grain process （IGP）. The influence of the process parameters on microstructures of the semi-solid A356 Al-alloy slurries were investigated， and formation mechanism and morphology controlling of the spherical primary α（Al） grains was discussed. The results show when the alloy temperature is 630 ℃， the preparing slurry is 4 kg， the IG size is 10 mm， dosage is 3.5 % （mass fraction）， and dumping temperatures（DTs） are at 611~617 °C， the mean diameters of the primary α（Al） grains in the semi-solid slurries can reach 40~75 μm and the shape factors can reach 0.82~0.89. When the IG size is 10 mm， the DT is at 613 ℃， and dosage is 2 %~4 %， the mean diameters can reach 45~82 μm and the shape factors can reach 0.78~0.88. With the alloy temperature and the DT decreasing or the dosage increasing properly， the primary α（Al） grain morphology is better. When Q_R =Q_A and Rh= Rc， as long as the DT is suitable， excellent semi-solid slurry can be produced. As a result of the IG melting， a large amount of dendritic fragments can become direct source of the primary α（Al） grains. Meanwhile， many under-cooled areas are formed， where abundant primary α（Al） grains are multiplied by heterogeneous nucleation.