The influence relative humility and temperature on water vapor transmission rate of polypropylene films with different tropisms were studied. The result indicated that the water vapor transmission rate of polypropylene films increases with temperature and relative humility; temperature has greater influence on water vapor permeability of polypropylene films than humidity; under the same temperature and relative humility, water vapor permeability of BOPP, OPP, and CPP films increases successively. It was concluded that vapor permeability of PP films decreases by stretch orientation.