Ultrasound sonication is one of the promising techniques to disperse the nanoparticles into a polymeric matrix thoroughly. In this study, the effects of ultrasonic parameters (amplitude%, time, and cycle of sonication) on the dispersion of 1 wt% nano-clay in polyester matrix were investigated. To disperse the nano-clay into the polyester matrix, sonication amplitudes of 40 and 80%; sonication times of 0.5, 1, and 2 h; and pulsed as well as continuous cycle acoustic irradiation modes were used. Compared with the unfilled glass fiber/polyester composites, the tensile strength and fracture toughness of glass fiber/polyester composites filled with nano-clay were improved with all sonication parameters. The ultrasonic parameters were of significant importance for dispersing nano-clay onto glass fiber/polyester composites and improving the interfacial properties of the composites. Sonication using an amplitude of 40% with pulse control mode of 0.5 cycle and time of 2 h resulted in maximum improvement in both ultimate tensile strength and fracture toughness of 22 and 33.4%, respectively, as compared to neat specimens.