This thesis is about the design of the gating system and selection of proper alloy for defects free (grate bar) casting. The gating system plays an important role in casting manufacturing process. The gating system has different elements like pouring cup, sprue, well, runner, riser, and ingates. The function of the gating system is to provide molten metal to the mould cavity through different gating system elements.  Casting is a metal shaping process which is used to produce a cast component. The casting process depends upon the material, type of pattern, mould and various techniques like sand casting, investment casting, die casting, squeeze casting and lost foam casting. The sand mould casting process is used in this report. The casting process is used for making small to large cast parts, complicated shapes, and precision parts, etc. Making a casting without defects is an important requirement for its strength. The effective and efficient design of the gating system is necessary for making defects free castings. There are various defects like shrinkage cavity, porosity, pinholes, blowholes and incomplete filling that may occur in sand casting. The simulation software like Magma Soft and Nova Flow Solid are used to predict the possible defects in the casting. The uses of the simulation improve product quality and increase productivity. It also helps to reduce the rejection rate by identifying and controlling defects. This work is done at AB Bruzaholms Bruk as part of master thesis work at Jönköping University, Sweden. The company provides all the necessary data for simulation purposes. The design of the gating system is finalized as per company requirements and needs. The research questions that have been answered in this report based on the following points. 1) What does zero-defect mean? 2) Which is the best design among the ones that are prepared and simulated? 3) Which is the best alloy combination for casting parts that give defects free casting and better fluidity and filling?