Fouling of heat transfer surfaces during subcooled flow boiling is a frequent engineering problem in process industries. It has been generally observed that the deposits in such industrial systems consist mainly of calcium carbonate (CaCO3), which has inverse solubility characteristics. This investigation focused on the mechanism to control deposition and the morphology of crystalline deposits. A series of experiments were carried out at different surface and bulk temperatures, fluid velocities and salt ion concentrations. It is shown that the deposition rate is controlled by different mechanism in the range of experimental parameters, depending on salt ion concentration. At higher ion concentration, the fouling rate increases linearly with surface temperature and the effect of flow velocity on deposition rate is quite strong, suggesting that mass diffusion controls the fouling process. On the contrary, at lower ion concentration, the fouling rate increases exponentially with surface temperature and is independent of the velocity, illustrating that surface reaction controls the fouling process. By analysis of the morphology of scale, two types of crystal (calcite and aragonite) are formed. The lower the temperature and ion concentration, the longer the induction period and the higher the percentage of calcite precipitated.