Abstract The cast primary dendritic structure is influenced by the solidification and crystallization processes, by the chemical composition, and casting parameters of the steel. There is a direct influence of the quality of the dendritic structure on the occurrence of both, surface or internal defects in products. The following parameters characterize the quality of the primary solidification structure of continuously cast slabs: the thickness of the surface zone of fine dendrites - the so called skin, and the uniformity of that, the consistent transition from the skin into the zone of columnar dendrites, the primary and secondary dendrite arm spacing, and the size of the central zone with equiaxed crystals. This work describes the quality evaluation of the primary dendritic structure for continuously cast slabs made of peritectic commercial steel continuously cast in a curved type plant PZO, at different casting rates ranging from 0,5 to 1,3 m/min. The following structural parameters were considered: the slab skin, the primary and secondary dendrite arm spacing in the crystallization zone, and the size of the central zone with equiaxed crystals. The obtained results led to the conclusion, that the tested steel types can be cast at casting rates 1,3 to 1,4 m/min only, if they have higher purity, and if higher quality of the following processing can be maintained.