The GLA domain, a common membrane-anchoring domain of several serine protease coagulation factors, is a key element in membrane association and activation of these factors in a highly Ca2+-dependent manner. However, the critical role of Ca2+ ions in binding is only poorly understood. Here, we present the atomic model of a membrane-bound GLA domain by using MD simulations of the GLA domain of human factor VIIa and an anionic lipid bilayer. The binding is furnished through a complete insertion of the omega-loop into the membrane and through direct interactions of structurally bound Ca2+ ions and protein side chains with negative lipids. The model suggests that Ca2+ ions play two distinct roles in the process: the four inner Ca2+ ions are primarily responsible for optimal folding of the GLA domain for membrane insertion, whereas the outer Ca2+ ions anchor the protein to the membrane through direct contacts with lipids.