Grey cast iron is frequently used in industry, due to good thermal conductivity, low price and good mechanical properties. Chemical properties and solidification conditions have significant influence on microstructure and mechanical properties development. Most common use of grey cast iron is in automotive industry and heating elements. The application in the casting of thinwalled castings requires careful elaboration of the technological casting process parameters due to the curving tendency and the burns appearance. Advantage of numerical simulations is in effective and fast calculation of pouring and solidification of castings. Economical advantage lays in fast change of more different product variations in short time. Experimental part of this paper has several versions of numerical simulation of casting process. First one is the original geometry, which has been used in regular casting process of thin-walled castings framework for fireplace doors. Other versions with optimizing geometry of the castings have been elaborated. Pouring process of all casting versions of EN GJL-200 quality was performed followed by investigation of casting quality and process parameters interaction. The influence of clamp position, ingates size and temperature distribution during casting solidification on microstructure development, hardness and tensile strength of grey cast iron EN GJL-200 casting samples was determined and compared with the results obtained by numerical simulation.