To improve the membrane, if CoAlPO4-5 was appropriately added, it could increase the rejection rate of metal ions compared with the PSf membrane. Besides, penetration rate would also increase, and the pressure applied could be lowered when the improved membrane was used. The optimum amount of CoAlPO4-5 added for Co : Al : P = 0.1 : 0.92 : 1.0 is about 5 wt %, while for the Co : Al : P = 0.08 : 0.93 : 1.0, this optimum value would shift toward a higher wt % because the latter has less active sites per unit weight of CoAlPO4-5. The hydrated radius of metal ion had great influence on the rejection rate. The larger the radius, for example, that of the +3 valence cations, the higher the rejection rate would exhibit, and the rate for Cd2+ was much lower because its radius was the smallest one. Moreover, although incompatibility existed in the improved membrane and the variation of applied pressure was limited, durability of this membrane under the operating conditions was good.