The study of magnetic effect on the precipitation of calcium carbonate was widely studied. The main objective was to prevent the mineral scales deposits on the walls of industrial or domestic equipments. Magnetic water treatment for scale prevention has been around and has been reported as being effective in many instances. This investigation focused on MF effect on CaCO3 in presence of foreign ions. Because of their abundance in natural waters Mg2 , SO4 2-, Na and Cl- were chosen for this study. It was shown that MF application reduces the induction time (tind) some either the water composition or the ionic strength (IS). With respect to reference solution, the effect of MF on tind is more pronounced in presence of foreign ions until IS= 0.02 M. Beyond this value, MF effect on tind become less important. Moreover, it was shown that MF effect on total and homogeneous precipitation ratios decreases with IS increasing. This inhibitory effect of magnetic treatment, with IS, is water composition-dependent. The presence of NaCl has the most effect on total precipitation whereas MgCl2 and Na2SO4 inhibit the precipitation of calcium carbonate in bulk of the solution.