Evolution on morphology of primary phase in semi-solid ZL101 Al alloy during isothermal holding in the solid-liquid area is investigated and the effect of processing parameters on the morphology is discussed. The results indicate that the morphology and grain size gradually become better and finer with the holding temperature decreasing under the different holding temperature. The morphology gradually comes into better and the grain size is still finer with prolonging of holding time at the same holding temperature but different holding time. Pouring temperature has the bigger effect on the morphology and the evolution of primary phase at the isothermal holding. The suitable processing parameters prepared semi-solid ZL101 Al alloy slurry are obtained.