Microstructure influences on damaging mechanisms in ferritic–pearlitic ductile cast irons (DCIs) were investigated. Four different ferrite/pearlite volume fractions were considered, and in situ scanning electron microscope observations were performed during step by step tensile tests in a microtensile holder. SEM observations allowed to identify an evident microstructure influence on ferritic–pearlitic DCIs damaging mechanisms and the key role played by graphite constituents. Phase volume fractions and distribution affect both crack initiation and crack propagation. The importance of the damage mechanism based on graphite nodules–matrix interface cracking, usually indicated as the most relevant, is only partially confirmed, depending on the DCI microstructure.