Epoxy resin/ montmorillonite (EP/MMT) composite was prepared via monomer insert in-situ polymerization. It was shown that the EP/MMT composites could be successfully synthesized from the raw materials only by one step, making it simpler than traditional in-situ intercalative polymerization method. The d001 spacing of montmorillonite was tested by XRD and increased to 4.30nm. The results showed that tensile strength and impact strength were improved by 98.11 and 93.69%, respectively. The glass transition temperature was increased by 17.3 °C.