The microstructure and interface characteristics of Mg2B2O5 whisker-reinforced magnesium matrix composites were investigated using optical microscopy, transmission electron microscopy and X-ray diffraction. It is found that the Mg2B2O5 whiskers have a twinned structure with the (202) as the twin plane and [010] as the growth direction of crystals. The MgB4O7 particles exist in some Mg2B2O5 whiskers. Equiaxial Mg2Si particles exist between the grain boundary of the matrix and the Mg2B2O5 whiskers. The MgO and MgB2 phase interface layers with different thickness are formed at the matrix-whisker interface during the vacuum-gas pressure infiltration process, which result from the interfacial reaction between Mg2B2O5 whisker and Mg matrix. The crystallographic orientation relationships between the Mg2B2O5 whisker and the interfacial reaction products are found to be [010]Mg2B2O5//[110]MgO and [110]MgO//[]MgB2, and (202)Mg2B2O5//(002)MgO and (002)MgO//(0001)MgB2, respectively. Finally, the factors that influence the microstructure of the Mg2B2O5 whiskers and the presence of the Mg2Si, MgO and MgB2 phases nearby the Mg2B2O5 whiskers were discussed.