Two kinds of commercial alumina compacts made by injection molding with different solid loading are studied. Solid loading is the key factor for the injection molding process and the properties of green body and sintered compacts. The results show that higher solid loading leads to better properties of the compacts such as density and fracture strength, however the viscosity of the feedstock would increase. Smaller sized alumina powder leads to a higher critical solid loading value. Macroscopic defects such as voids and cracks could be introduced into the injection molding samples if the molding variables are not optimized. The properties of sample are improved by adjusting the variables such as injection pressure, holding pressure, holding time and barrel temperature.