This study investigates the initiation of martensite/ferrite (M/F) interface damage in dual-phase (DP) steels, a critical failure mechanism that remains poorly understood. Through an integrated experimental-numerical approach, the research examines the hypothesis that substructure boundary sliding triggers and dominates M/F interface damage initiation, accompanied by apparent martensite plasticity. The mesoscale morphology and prior austenite grain reconstruction serve as modeling inputs, and a multi-scale framework is used to predict interface damage initiation. The predicted damage initiation sites align well with those identified from in-situ experiments, confirming the key role of substructure boundary sliding. Additionally, the study finds that M/F interface damage initiation strongly correlates with low M/F strain partitioning rather than the commonly accepted strong M/F strain partitioning. This fundamental understanding is crucial for optimizing the microstructure of DP steels to balance strength and ductility. The research highlights the importance of incorporating the substructure boundary sliding mechanism in models to accurately predict the behavior of martensite islands and the initiation of M/F interface damage.This study investigates the initiation of martensite/ferrite (M/F) interface damage in dual-phase (DP) steels, a critical failure mechanism that remains poorly understood. Through an integrated experimental-numerical approach, the research examines the hypothesis that substructure boundary sliding triggers and dominates M/F interface damage initiation, accompanied by apparent martensite plasticity. The mesoscale morphology and prior austenite grain reconstruction serve as modeling inputs, and a multi-scale framework is used to predict interface damage initiation. The predicted damage initiation sites align well with those identified from in-situ experiments, confirming the key role of substructure boundary sliding. Additionally, the study finds that M/F interface damage initiation strongly correlates with low M/F strain partitioning rather than the commonly accepted strong M/F strain partitioning. This fundamental understanding is crucial for optimizing the microstructure of DP steels to balance strength and ductility. The research highlights the importance of incorporating the substructure boundary sliding mechanism in models to accurately predict the behavior of martensite islands and the initiation of M/F interface damage.