We investigated the mechanism of CaSO4 and CaHPO4 scaling on nanofiltration membranes by observing the flux decline behavior. It was found that CaSO4 fouling contributed a greater resistance than CaHPO4 fouling under the same operating conditions. CaSO4 fouling was characterized by reversible cake growth caused by both bulk and surface crystallization at a lower operating pressure, and by surface crystallization at a higher operating pressure. CaHPO4 fouling was characterized by both irreversible pore/surface adsorption and reversible cake growth through surface crystallization regardless of operating conditions. These findings indicate that both fouling solutes and operating parameters should be considered when evaluating inorganic fouling of a nanofilter.