Cooling rate plays an important role in the formation of thin wall ductile iron microstructure due to their thickness, which is 3 mm below based on Stefanescu. Cooling rate is closely related to casting design and determines the microstructure. This paper discusses the effect of casting design modification to casting yield and microstructures. Modification was made on a patented design used previously to produce thin wall ductile iron plates. The design was minimized and casting simulation was used to analyze castability of the design. After that, the design were casted in several pouring temperatures. Improvement was made to casting design based on the failure during the experiment. Casting process took place after simulation analysis. The casting product was fully casted as shown by the simulation. The casting yield has improved to 28%. When all plates were examined for microstructure, the result showed that all the microstructure of the plates was not graphite in ferrite matrix as occurred in the patented design but it was graphite in pearlite matrix.