Urea-formaldehyde (UF) resins modified by montmorillonite(MMT) were prepared for coating materials of controlled-release fertilizers. The influences of MMT on the mechanical properties and fertilizer-permeation performance of UF resin were studied. The mechanical properties of modified UF resins are enhanced by the addition of MMT. The shear strength of modified UF resin with 3% MMT increases by nearly 20% and reaches the maximum. The urea permeation rate of modified UF resin membranes decreases with the increasing of membrane thickness and increases with the increasing of MMT content.