Although the magnesium sheet shows promising applications in automotive, aerospace, and electronic housing industries, its development is still limited due to the present manufacturing barriers. During a production of magnesium plates, as precursors for sheet rolling, the frequently overlooked factor is the effectiveness of commonly used mold materials in controlling the cooling rate thereby increasing or decreasing the solidification time, the riser size and casting defects. The objective of this study was to cast magnesium alloy plates with different cooling rates to determine the effect of cast structure on the resulting sheet properties and microstructure. For this purpose a 6 mm thick AZ31B plate was cast by gravity, vacuum and twin roll casting methods in permanent (graphite and metal molds) and sand molds (silica, carbo ceramics, and olivine). The microstructure of the plate was studied and secondary dendrite arm spacing (SDAS) was measured utilizing quantitative metallography techniques and image analysis. The obtained results, including the relationship between SDAS and cooling rate were compared with the literature data. As the cooling rate increased, the SDAS decreased and microstructure became finer. Because of this work, magnesium plates were produced for rolling purposes that have different cast structures, with SDAS’s ranging from 4.5 – 40 micron.