Within this thesis, cast stainless steel impeller with porosity defects was analysed. The first part gives overview of stainless steel and the division and characteristics of every group. After that, casting methods are listed with the methods that are used within this thesis explained in detail. In the experimental work, an analysis of the casting was conducted at first. Study of chemical composition and microstructure characterization showed that material was austenitic stainless steel AISI 316L. After that, CAD model of existing casting was designed with belonging gating and feed systems. Existing casting was casted with manual sand casting in green sand and with pressurized gating system. Simulation of the process was conducted in simulation software program ProCAST. Analysis of the simulation results showed that casting had defect in high thickness area because of insufficient feeding with melt during the solidification. By comparing the results from simulation and from existing casting it was concluded that same defects appeared in the same area which verifies simulation results. As an improvement proposal, chill was installed onto the casting and pressurized gating system was designed. Chill should have enabled directional solidification toward the riser and pressurized gating system should have enabled reliable fill of the mould cavity. After simulation in simulation software program ProCAST was conducted and results analysed, casting was correct and without defects.