A porous film, a separator including the porous film, an electrochemical device including the porous film, and a method of preparing the porous film are provided, wherein the porous film includes: small-diameter fibers having an average diameter of about 50 nm or less; and large-diameter fibers having an average diameter of about 100 nm or greater, wherein an amount of the large-diameter fibers is about 5 wt % to about 60 wt % based on a total weight of the small-diameter fibers and the large-diameter fibers.