Within this thesis six aluminium alloy EN AW-5083 slabs cast with the semi-continuous, vertical direct water cooling process (Direct Chill) have been evaluated. It has been determined how the cooling rate influenced size and number of grains per unit area and tensile properties (tensile strength, yield strength and elongation) of slabs. In the experimental part of this work examinations of microstructural and tensile properties on specified places withing the cross sectioned 300 mm thick place( „slice“ ) on the beginning and the end of the cast slabs were performed. The base for determining influence of the cooling rate was statistical examination plan („latin square“). Microstructure examination in the „slices“ it revealed the unequal distribution of equiaxed grains, without the columnar crystal zone. Graphic figures showed influence of different sources of variability and effect of cooling rates on growth and number of grains distribution. Number of grains grows from center to the rears of ingots. Change of microstructural properties is followed by the change of tensile properties. By statistical processing of obtained results it has been established that the slice height (ingot thickness) is the most significant variability source. Relatively high correlation coefficients between the slices for the beginning and the end of examined ingots for grain numbers and tensile properties (from 0,66 to 0,77) indicate relatively acceptable chemical, microstructural and mechanical homogeneity of the investigated cast slabs. Obtained results reference the relative consistency and repeatability of the melting process, melt treatment and casting of the EN AW-5083 aluminium alloy slabs.