Magnesium alloys due to their low density and high strength-to-weight ratio are promising material for the automotive and aerospace industries. Many elements made from magnesium alloys are produced by means of sand casting. It is essential to investigate impact of the applied mould components on the microstructure and the quality of the castings. For the research, six identical, 100x50x20mm plates has been sand cast from the Elektron 21 magnesium casting alloy. Each casting was fed and cooled in a different way: one, surrounded by mould sand, two with cast iron chills 20mm and 40mm thick applied, another two with the same chills as well as feeders applied and one with only the feeder applied. Solid solution grain size and eutectics volume fraction were evaluated quantitatively in Met-Ilo program, casting defects were observed on the scanning electron microscope Hitachi S3400N. The finest solid solution grain was observed in the castings with only the chills applied. Non metallic inclusions were observed in each plate. The smallest shrinkage porosity was observed in the castings with the feeders applied.