This work was performed to investigate proper condition of coagulation treatment as UF process pretreatment that consider UF permeate flux and residual Al concentration. The coagulant used an alum as Al2O12S3·16H2O and the experiment was tested in adjusting conditions such as alum dose, flocculation time and pH of seawater. Consequently, higher alum dose lead to elevation of UF permeate flux while residual aluminium also increased in condition of pH 8.0. The most suitable condition which has a good permeate flux and low residual aluminium, In this works, was alum dose of 0.025 mM as Al and pH 6.5. The flocculation time reduced UF permeate flux.