Blood coagulation factor VIII (FVIII) can bind calcium ions and ion-protein interactions appear central importance for both their structure and function in coagulation cascade. However, the mechanism and details of how calcium dependent structure change of proteins to fulfill their function remain to be fully defined. In this work, PeakForce Quantitative Nanomechanics (PF-QNM) mode atomic force microscopy (AFM) was used to map the topography and mechanical properties of FVIII with single protein resolution under different calcium concentrations. The obtained nanomechanical spectroscopy showed that calcium ions play dual roles in the chain association and structural flexibility of FVIII. Low concentration of calcium ions prefer to bind isolated chains and increase their mechanical properties, whereas they link the heavy and light chains to keep the protein re-association under higher ions concentration. Our results provide a novel insight into the mechanistic details of the metal ions on the stability and function of blood clotting proteins. (C) 2019 Elsevier Inc. All rights reserved.