The main parameter required to characterize the rheological behavior of semi-solid metal, SSM, slurries is the "viscosity". Since the viscosity depends on the microstructural characteristics of the SSM slurries and it is also a measure of their rheological behavior, viscometry may be employed to study the rheology and structure of SSM alloys. This is much faster and less expensive microstructural characterization method than quantitative metallography which is a time consuming operation and requires a highly skilled operator. Furthermore, it may be used as an on-line quality check in production of rheo-billets used as feedstock for near net shape manufacturing routes. Al-Si 356 foundry alloy with different morphologies of primary alpha-Al particles was tested at different initial applied pressures of 5-11 KPa, using parallel plate compression viscometery. The resulting strain-time graphs were further treated mathematically to calculate the viscosity of SSM billets. The viscosity was then attributed to the microstructure and primary phase flow during compression. It was shown that the dendritic primary alpha-Al structure has the highest viscosity number which is almost three orders of magnitude greater than those for a globular morphology. Such difference reduces to one order of magnitude when the rosette morphology is compared to that of globular structure. This study has shown the validity and reliability of the "parallel plate compression viscometry" method in characterizing the microstructural evolution of rheocast SSM billets and highlights the correlation between the viscosity numbers and the resulting microstructures cast at different pouring temperatures.