Human RAD51 protein (HsRad51)-promoted DNA strand exchange, a crucial step in homologous recombination, is regulated by proteins and calcium ions. The activator protein Swi5-Sfr1 and Ca2+ ions stimulate different reaction steps and induce a perpendicular orientation of DNA bases in the presynaptic complex. To investigate the importance of base orientation in the strand exchange reaction, we examined the Ca2+ concentration dependence of strand exchange activities and structural changes in the presynaptic complex. Our results show that optimal D-loop formation (strand exchange with closed circular DNA) requires Ca2+ concentrations greater than 5 mM, while 1 mM is sufficient for strand exchange between two oligonucleotides. The structural change, which is evidenced by an increase in fluorescence intensity of poly(dεA) (a poly(dA) analog), reaches a plateau at 1 mM Ca2+. Meanwhile, the linear dichroism signal intensity at 260 nm, which is indicative of rigid perpendicular DNA base orientation, requires >2 mM Ca2+ for saturation and thus correlates with the stimulation of D-loop formation. Therefore, Ca2+ exerts two different effects. Thermal stability measurements suggest that HsRad51 binds two Ca2+ ions with KD values of 0.3 mM and 2.5 mM, implying that one step is stimulated by one Ca2+ bond and the other by two Ca2+ bonds. We further discuss the parallels between Mg2+ activation of RecA and Ca2+ activation of HsRad51.