To examine the effects of different coagulants on membrane fouling in the coagulation/aid-coagulation/ultrafiltration process for high turbidity surface water purification, Malvern laser particle size analyzer and micro-scale ultrafiltration unit were used to investigate membrane fouling caused by the flocs formed by coagulants (i.e. aluminium polychlorid (PACl), ferric trichloride (FeCl₃)) and aid-coagulant (polyacrylamide (PAM)). The results indicated that, with the constant dosage of combined coagulants (PACl=FeCl₃=3 mg/L), the PAM dosage of 0-0.4 mg/L rendered the d50 of flocs exhibited an obvious increase (eventually reached to 1 000 μm) compared to those with single coagulant dosage of PACl (3-370 μm) or FeCl₃(360-420 μm). With the dosage of PAM increased, specific membrane flux increased (PACl:0.56-0.64; FeCl₃: 0.71-0.76) and cake resistance decreased (PACl:0.90×10^-11-0.52×10^-11 m^-1; FeCl₃:0.47×10^-11-0.28×10^-11 m^-1). However, when the dosage of PAM increased to 1.0 mg/L, the membrane flux decreased significantly to 0.48 while cake resistance increased to 1.55×10^-11 m^-1. An optimum flocculant dosage (0.4 mg/L) that caused the minimum membrane fouling was confirmed. Furthermore, the results obtained from Nanoseries Zetasizer and scanning electron microscopy (SEM) demonstrated that cake layer was the dominant fouling mechanism under different conditions and membrane pore blocking was severe with higher PAM dosage.