Three ADI specimens with silicon contents of 2.4, 2.9 and 3.3 in wt% were used in this program. The effect of graphite size on the features of ADI were also used in specimens with 2.4wt%Si and 3.3wt%Si. As a consequence, the effect of silicon and graphite size on the features of ADI should be studied. As revealed in experimental results and discussions, the features of ADI are divided into two temperature regimes. In the first regime, at about R.T. to 300, the feature is due to the dynamic strain aging. The cause of dynamic strain aging is attributed to the acicular ferrite and the dislocation density in ADI microstructure should play an important role on the occurrence of flow instability. The second regime is at about 300.degree.C to 500.degree.C, the feature is due to the effect of carbide. Otherwise, the flow stress increases and elongation decreases as the content of silicon increases. This effect of silicon addition on the mechanical properties is attributed to the more casting defects with high silicon content. Another results indicate that the flow stress and elongation both increase with smaller graphite size. The finer microstructure of ADI is the reason for this effect of graphite.