Good dispersion of the nanoparticles into the polymer is considered a critical issue, as it can provide higher strength and stiffness while poor dispersion is seen to decrease those properties. In the present work, the effect of three ultrasonic parameters (amplitude, time and cycle of sonication) on sonication technique for dispersing 1 wt.% nano-clay in polyester matrix was investigated. To disperse the nano-clay into the polyester matrix, sonication frequencies of 40% and 80%, sonication times of 0.5, 1 and 2 hours and pulse of 0.5 and 1 cycle were used. The effect of these ultrasonication parameters on water barrier and impact behavior of unfilled and filled glass fiber (GF)/polyester with nano-clay under dry, distilled and seawater conditions was studied. Results showed that, water absorption of nano-filled composites dispersed with all sonication parameters is lower than that of unfilled glass fiber/polyester composites immersed in distilled and seawater. Nano-clay filled GF/polyester composites showed an improvement in impact resistance under dry, distilled and seawater conditions with all sonication parameters. Among the used sonication parameters; time of 2 hours, amplitude of 40% and 0.5 cycle was found as the best parameter which resulted in the maximum enhancement in impact resistance, due to the addition of nano-clay to GF/polyester, of 8.2%, 14% and 19.6% under dry, distilled water and seawater conditions, respectively. Nonlinear minimization approach was exploited using MAPLE commercial software in order to find the suitable fit to the models of Fick and Langmuir. Diffusion coefficients for different sonication times were computed.