Periclase crystal size of magnesia is an important characteristic parameter for magnesia raw materials, it will strongly influences the properties of these raw materials, especially the corrosion resistance. In this thesis, two methods for the crystal size measurements, the grid method ASTM E 112-96 (Reapproved 2004) and the lineal analysis are compared. Both methods show credible and accurate measurement results. The grid method is recommended as the general method for routine measurements in test work due to its efficiency. Lineal analysis can be used as an auxiliary method for more accurate requirements. To identify crystal boundaries clearly, etching of the polished sections with HNO3 or H2SO4 solution was applied. For the results of the grid method, the statistical analyses of crystal size measurements of fused magnesia differed for various brands. The crystal size of the blend material of magnesia clinker and fused magnesia always shows a higher standard deviation than that of the normal type of fused magnesia raw material. The secondary phases in the magnesia samples could be identified by chemical analyses and mineralogical characterization. For the samples investigated here mainly monticellite, merwinite, dicalcium silicate and tricalcium phosphate have been identified. The amount of the secondary phases is determined by the raw material purity.