The cross flow type ultrafiltration was carried out to evaluate the relation between flux and coagulation state using the results of zeta potential, flux reduction ratio, volumetric sedimentation ratio, the turbidity reduction ratio in Jar test. The concentration of kaolin suspension used in this study was 1000 mg/l. The ion concentration of the raw water was controlled by changing mixing ratio of distilled water and tap water, or with adding NaCl and CaCl₂ solution. As the dispersion state of kaolin suspension was good, the flux of ultrafiltration hardly decreased because of the large negative charge of suspended solid and shearing force under cross flow. If the dispersion state of kaolin suspension was not good, the flux of ultrafiltration was getting small by destabilization of dispersion state. This small change of coagulation state gives large influence on the flux. On the other hand, the cation or coagulant dosage makes coagulated flocs and the resulted large floc and shearing force under cross flow obstructs the cake formation on membrane.