Let ∇ be a symmetric connection on an n-dimensional manifold Mn and T∗Mn its cotangent bundle. In this paper, firstly, we determine the infinitesimal fiber-preserving projective(IFP) transformations on T∗Mn with respect to the Riemannian connection of the modified Riemannian extension g˜∇,c where c is a symmetric (0, 2)-tensor field on Mn. Then we prove that, if (T∗Mn, g˜∇,c) admits a non-affine infinitesimal fiber-preserving projective transformation, then Mn is locally flat, where ∇ is the Levi-Civita connection of a Riemannian metric g on Mn. Finally, the infinitesimal complete lift, horizontal and vertical lift projective transformations on (T∗Mn, g˜∇,c) are studied.
