The nominal pore size 0.2 ${mu}m polytetrafluroethylene(PTFE, Satorius Co,) membrane was used for the filtration of 0.1 wt% kaolin, bentonite, yeast, and starch solution, respectively. After filtration, the membrane was cleaned and the pore size was measured by liquid displacement method(LDMl using water/iso-butanol system. The pore size for new PTFE membrane evaluated by LDM was comparable with those of which were measured by mercury porosimetry and scanning electron microscope. The membrane pores were severely fouled, and constricted to below 0.3 ${mu}m in the cases of bentonite and starch solution which contained smaller particles than pores. However, in the case of kaolin - solution, only some parts of membrane pores above 0.35 ${mu}m were slightly fouled. Hence, the phenomena of membrane fouling could be identified quantitatively by the evaluation of pore size using LDM.