Thermo-responsive poly (N-isopropylacrylamide) (PNIPAM) polymer has been synthesized by free radical addition polymerization method. The chemical structure of the synthesized polymer has been clarified by FTIR spectroscopy and H-1 NMR analyses. Turbidity test shows that the synthesized polymer exhibits thermo-responsive properties, depending on change in temperature. Its lower critical solution temperature (LCST) value is measured as 31 degrees C by DSC analysis. The PNIPAM polymer is then applied onto the cotton fabric in two different concentrations using double-bath impregnation method. Change in hydrophilic character of the fabric, which is temperature dependent, has been revealed by drop and absorption capacity tests, contact angle measurement and surface energy calculation. The test results show that the fabrics exhibit thermo-responsive behavior. Their hydrophilic character is turned to the hydrophobic character above LCST of the polymer. The water vapor permeability of the polymer treated fabrics at temperatures above LCST increases as compared to the untreated fabric due to the increase in fabric porosity. Below LCST, water vapor permeability is increased because of the increasing hydrophility. Consequently, it is concluded that the water vapor permeability of the fabrics can be controlled by changing the temperature-dependent hydrophilic/hydrophobic characteristic and porosity, resulting from swelling or shrinkage of the polymer molecules.