The permeation flux of water vapor in membrane distillation is affected by membrane properties, the difference in the vapor pressure between the opposite sides of the membrane, and the operational conditions. Since the vapor pressure is a function of the concentration and the temperature, the vapor flux through a membrane is decreased with increasing concentration and decreasing temperature of the salt solution. Permeation experiments with a PTFE membrane (80 mu thick, 0.2 mu m pore diameter, 0.75 porosity, 2.38 x 10(-3) m(2) effective surface area) were conducted by using a batch cell, which had two reservoirs stirred by magnetic bars, separated by a membrane, and thermostated by hot water and cool water circulated in respective jackets. The temperature of the hot water was ranged from 308 to 373 K, and the temperature of the cool side was kept at 288 K. The stirring rate was varied from 200 to 800 rpm. The concentration of aqueous lithium bromide solution was ranged within 0 to 55 wt%. The water flux was obtained by the moving rate of the water meniscus in the capillary connected to the sealed cell. The permeate flux was affected by the thermal and concentration boundary layers. The analogy analysis for the thermal and concentration boundary layers was applied to the permeation mechanism, and well explained the effects of the stirring rate and the difference in the temperature on the permeation flux. The thickness of the thermal boundary layer was found to be larger than that of the concentration boundary layer, and both layers were hardly negligible under the conditions of this work.