Factors affecting oil separation from surfactant-free dispersed oil-water mixture in a mixing tank by a poly(tetrafluoroethylene) (PTFE) membrane separator unit were investigated. The typical unit consisted of a sheet of porous PTFE membrane, a PTFE filter paper and an acrylic perforated plate. The separation process included three steps in series: 1) attachment of dispersed oil droplets to the membrane surface, 2) penetration of the attached oil into and through the porous space within the membrane and the filter paper, and 3) release of the oil from the separation unit. The resistance of step 2) was negligibly small compared with the other two steps. The ratio, a, of the oil droplet amount separated by the separation unit to that colliding with the membrane surface was very small, i.e., 3 % at most within the experimental conditions. The effect of oil viscosity was significant and the ratio a was inversely proportional to the oil viscosity.