Polypyrrole (PPy) composite films with different contents of Fe3O4 were prepared by in-situ polymerization of pyrrole in aqueous solutions. The dependence of dc current changes on the response of the samples exposure to NH3 vapor has been investigated. The results shows the composite films are more stable than the pristine ones after being exposed to NH3 vapor. Meanwhile, the response time was reduced with increasing the Fe3O4 content in the films. The results might be originated from the structural changes in the PPy films caused by the addition of Fe3O4.