Hypereutectic Al-20Si (mass fraction) alloy hollow tubes were prepared by centrifugal casting under an electromagnetic field (EMF). Effects of EMF on the solidification microstructure of the alloy and morphology of primary Si phases were investigated. Results show that primary Si phases in the solidification microstructure were refined after applying EMF compared with the microstructure of the Al-20Si hypereutectic alloys by conventional centrifugal casting. The segregation layer width of Si phases in the outer of the tube decreased gradually with the increase of the EMF intensity. The morphology of the Si phases in the outer of the tube changed from typical pentagram into dendrites grown along the heat flow direction. The phenomena of dulling and congregation of the Si phases in the inner of the tube were found due to EMF stirring. The size and aspect ratio of the primary Si phase decreased little by little with the increase of EMF intensity. The electromagnetic stirring destroyed the radial configuration of Si phases, eutectic Si phases distributed randomly, and the eutectic lamina spaces in the eutectic cell increased gradually.