A novel extrusion process was used in order to obtain flame retardant polymer nanocomposites. Ultrasonic waves were applied on-line during single screw extrusion process coupled to a special static mixer die, which promotes extensional flow. The effect of on-line ultrasound application on two polymer matrices (high density polyethylene, HDPE and polypropylene PP), halogen free flame retardant additives and an organo-clay was thoroughly studied. For each polymer matrix, different halogen free flame retardant additives were employed. The morphology, thermal stability, fire behavior by UL94, as well as mechanical and rheological properties were investigated. Morphological studies demonstrated that the novel extrusion process is able to reduce the size of non-halogen flame retardant additive particles and improves their dispersion and distribution in both polymer matrices, with respect to traditional extrusion processes (twin screw). This effect had a great impact on the flame retardant and mechanical properties of the materials. According to UL94 results the materials obtained with ultrasound system reached V2 classification. The addition of an organo-clay at specific content and the use of ultrasound system demonstrated that it is possible to reach an optimal V0 classification and to reduce the flame retardant additive content greatly, -40% in the case of HDPE and -30% in PP, with respect to twin screw extrusion process. Mechanical properties, Izod impact resistance, tensile strength, strain at break and tenacity were greatly increased. Rheology test performed in simple shear and linear oscillatory shear flow modes evidenced the enhanced particle dispersion and finer particle morphology which produced larger values of the moduli and deviations from the semicircular shape observed in the Cole-Cole diagram as compared to the system with no ultrasound application. Keywords: Ultrasound system, static mixer die, flame retardant