Abstract: With branched agent as the initiator of hyperbranched polyamide 6 (PA6), with organic modifier modified organic montmorillonite (O-MMT) which has the terminal carboxyl group, O-MMT/hyperbranched PA6 composites were prepared via in situ ring-opening graft polymerization. The grafting condition of hyperbranched PA6 and O-MMT, dispersing morphology of O-MMT in matrix were characterized by FTIR and TEM, and the effects of O-MMT on crystallization properties, melt flow properties and mechanical properties of hyperbranched PA6 were studied. Results show that O-MMT is exfoliated and intercalated dispersing in hyperbranched PA6 matrix, and chemically bonds with PA6 molecular chains to form crosslinked network structure, which makes the melt flow index of O-MMT/hyperbranched PA6 composites decrease sharply compared with hyperbranched PA6. In addition, O-MMT can make the α-crystal of hyperbranched PA6 disappear, and make crystallization degree of hyperbranched PA6 with only γ-crystal decrease. However, with the increase of O-MMT content, the strength of O-MMT/hyperbranched PA6 composites increase gradually, the tensile strength increase from 38.4 MPa to 60.8 MPa, the toughness of composites also has been greatly improved, the elongation at break increases from 2.1% to 70.1%, the un-notched impact strength increases from 20.3 kJ/m² increase to 291.8 kJ/m² sharply.