Abstract: The failure mechanism of graphite in ferrite ductile iron under tensile and impact action was studied by in-situ microscopic observation. The influence of morphology and distribution of graphite nodules on the micro-mechanism of crack initiation and propagation was analyzed. The results show that the graphite nodules separated from the matrix when the graphite nodules uniformly distributed and the spacing was larger than the average graphite size under tensile action, and there were radial cracks in the graphite nodule or cracks throughout the whole graphite. When graphite nodules showed aggregative distribution and spacing was smaller than the average size of graphite, the cracks between the graphite nodules and the matrix connected to form larger cracks. Under impact loads, there were complementary failure mechanisms such as "onion-like" cracking and internal radial cracking in graphite nodules. Lamellar tearing was observed in irregular graphite nodules under tensile and impact loads. The rapidly closed austenite shell could keep the graphite nodule rounded and the surrounding ferrite grains evenly distributed; slow closed austenite shell resulted in graphite distortion and uneven distribution of ferrite grains.