The novel polyurethane/montmorillonite (PU/MMT) nanocomposites based on poly (propylene oxide) glycol (POP), 4,4'-diphenymethylate diisocyanate (NIDI), 1.4-butanediol (1,4-BD) and MMT has been synthesized using a one-step direct polymerization-intercalation technique by twin-screw extruder. Its structure and thermal properties are characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM) and High-resolution electron microscopy (HREM), Fourier-transform infrared spectroscopy (FTIR) and Thermogravimetry analysis (TGA), respectively. The results of XRD and HREM analyses show that the silicate layer is well dispersed in PU matrix and this mesostructure can be considered as a delaminated nanocomposites. The TGA analysis indicates that the thermal stability properties of the PU/MMT nanocomposites are increased slightly compared with the pristine PU, due to the increase of the char residue. The mechanical and flammability performances are examined by electronic Universal Tester and Cone calorimetry, respectively. The layered silicate, which acts as a high aspect ratio reinforcement, enhances tensile strength of the PU. Specifically, there is a 25% increase in the tensile strength of PU nanocomposites containing 4 wt.% MMT compared with that of pristine PU. However, the elongation at break of PU/MMT nanocomposites is lower than that of pristine PU. The loading of MMT leads to the remarkably decrease of heat release rate (HRR), contributing to the improvement of flammability performance.