The effects of operating temperature and characteristics of membranes on the flux and the conductivity of permeate in membrane distillation were investigated. 3.5 wt% NaCl aqueous solution was used as a feed and hydrophobic microporous membranes were used. In spite of the variation of operating conditions, the NaCl was almost rejected. Both the pore size and the surface energy of membrane were inversely proportional to the range of operating conditions(i.e. upper limit of operating temperature). Both the flux and the permeate concentration were not affected by polymeric material and pore size of membranes. Although the air gap on the permeate side increases the efficiency of heat utilization, however, the mass transfer resistance also increases.