The present invention concerns a process for preparing a polymer nanocomposite composition, the process comprising: a) mixing a melted polyamide of inherent viscosity under 1 and a nanofiller to disperse the nanofiller in said polyamide; and b) subjecting the previous mixture to polymerization conditions to polymerize the polyamide and to form the polymer nanocomposite composition. Advantageously the inherent viscosity of the polyamide is under 0.9 and preferably between 0.4 and 0.8. Step a) is carried out in an extruder or a mixer. Advantageously a mono or twin-screw extruder is used. Step b) could be made either in melted state or in solid state. It is easier to make it in the same apparatus as step a). Should step a) is carried out in an extruder, step b) is made in the same extruder. Polymerization of step b) can be carried with a catalyst and/or by having the extruder zones in which step b) takes place to operate under vacuum.