The effect of inoculation efficiency and casting thickness on the final as cast structure of ductile iron castings was investigated. Four stages medium size ,testing cluster weighting 56 Kg. was utilized to determine the required conditions for production of as cast ferritic structure in thin section castings. Each stage contained wedge shaped specimen with variable thickness from 2mm up to 25mm. Time elapse between inoculation and complete filling of each stage varied from 14 seconds for the first level to 56 seconds for the fourth level. Low magnesium ferro-silicon alloy 5% Mg(VL 63) was used as inoculant with average grain size 3-5mm. Inoculation was performed directly inside the mould utilizing special reaction chamber located in the runner. Microstructure observation and hardness tests were carried out on the obtained as cast structure of the four stage wedges. The chilling depth in each level was evaluated. A relation between time elapse from inoculation till solidification start and final as cast structure was established. The impact of casting thickness on the as cast structure was found to be less than that of inoculation efficiency. Ferritic and Pearlitic grades of spheroidal graphite cast iron can be obtained by control of inoculation parameters in case of inmould process