Process industry remains skeptical of the effect of scale prevention in a water pipe by magnetic treatment despite its long history. In this study, mechanism of scale prevention by "alternating" magnetic treatment was investigated. Using suspensions of non-magnetic colloid particles (TiO 2 or CaCO3) dispersed in an electrolytic solution, we investigated zeta potential changes of the particles by imposition of alternating magnetic field with a specific frequency. Consequently, we found that the specific frequency treatment (several kHz) changes the zeta potential of particles drastically with an inversion of sign. This phenomenon obviously depends on the frequency. It is considered to be due to the specific adsorption of anions, that were easily adsorbed, in the solution onto the particles interface. From the experimental results, we concluded the mechanism of scale prevention by the treatment as follows: Normally, a surface of inner wall of a water pipe has negative charge and surface of scale particles in water has positive charge. So the particles are attracted by the water pipe and are attached to it. After the treatment, surface potential of the particles shifts to negative side, then repulsive force is formed between the particles and the wall.