An amino-functionalized montmorillonite (APTMS-MMT) was prepared by the grafting of 3-aminopropyltrimethoxysilane (APTMS) on the surface of MMT via the ultrasonic synthesis process and characterized by a variety of techniques: FT-IR, thermogravimetic analysis (TGA), particles size analysis and zeta-potential measurement. The results showed the size and size distribution of APTMS-MMT particles were decreased, and the zeta-potential of particles was increased obviously via the ultrasonic synthesis process. The particles of 30% APTMS-MMTUS (MMT modified with 30 wt% APTMS with ultrasonic synthesis process) had a z-average diameter of about 500nm and a polydispersity index of 0.2. The resultant 30% APTMS-MMTUS was dispersed uniformly and stably in water. The poly(acrylic acid) (PAA)/APTMS-MMT multilayer films were grown through layer-by-layer (LBL) deposition of PAA and APTMS-MMT. SEM results indicated that the ultrasonic synthesis of APTMS-MMT increased dispersability of clay sheets at high loadings. The thermal stability and mechanical properties of PAA/APTMS-MMT composites were investigated by TGA and tensile test respectively. The results showed the ultrasonic synthesis of APTMS-MMT enhanced the thermal stability and mechanical properties of PAA/APTMS-MMT composites significantly. PAA/30% APTMS-MMTUS composite displayed 3 times higher strength and 6 times higher Young's modulus when compared with pure PAA polymer. (C) 2011 Elsevier B.V. All rights reserved.