The application of a polymer film coated fertilizer can increase nutrient utilization efficiency and reduce environmental pollution, and polymer latex is an environmental friendly coating material with a promising future. However, because of the hydrophilicity of the film formed from polymer latex, it is difficult to achieve film coated fertilizer with a long release duration and a low consumption of the coating materials. Through spray coating in a fluidized bed, the sandwich structure of a composite film, i.e., polymer film/graphene oxide membrane/polymer film, was coated on KNO3 granules. Because of the longer diffusion path for the hydrated K+ and NO3- diffusing within the graphene oxide membrane and the slower diffusion rate caused by cation-pi interactions between the hydrated K+ and graphene oxide, the release duration of the film coated KNO3 fertilizer was extended. For the pure polymer film coated KNO3 granules, the release duration was 24 days, while the release durations of the composite film coated KNO3 granules were extended to 26, 29, 34 and 38 days when the amounts of graphene oxide in the coating were 0.18%, 0.72%, 1.43% and 2.86% of the film, respectively. It is shown that graphene oxide has great potential in applications in the field of controlled release fertilizers. (C) 2016 Elsevier B.V. All rights reserved.