The polymethacrylimide/organo-montmorillonite (PMI/OMMT) foams were prepared from acrylonitrile (AN) and methacrylic acid (MAA) by in situ intercalation polymerization of monomers and free heat foaming. Azobisisobutyronitrile(AIBN) was acted as initiator and OMMT was used as filler. The chemical structures of OMMT were characterized by Fourier transform infrared (FT-IR) and X-ray diffraction (XRD). The thermal properties, combustion properties, morphologies and mechanical properties of the PMI/OMMT foams were investigated in details by thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), vertical burning tests, limit oxygen index (LOI), scanning electron microscopy (SEM) and universal testing machine. The FT-IR and XRD results show that cetyltrimethylammonium bromide (CTMAB) is intercalated into the layers of sodium montmorillonite (Na-MMT) and the interlayer spacing is increased from 1.25 nm to 2.20 nm. The results of TGA and DMA demonstrate the prepared PMI/OMMT foams possess excellent char forming capability and higher glass transition temperature (Tg). The LOI values of PMI/OMMT foams are gradually increased with the increase of AN content. The SEM tests indicate that the prepared PMI/4OMMT foams are consisted of the honeycomb structure with the diameter of 0.1～0.4 mm. Meanwhile, the obtained PMI/OMMT foams present superior mechanical properties, for example, the tensile, flexural and compressive strength of the PMI60/4OMMT foam are 1.45 MPa, 2.11 MPa and 3.15 MPa, respectively.