We present a novel approach for the preparation of thermotropic liquid crystalline copolyester (TLCP)/multi-walled carbon nanotubes (MWCNTs) composites via in situ polymerization. A two-stage polycondensation procedure was employed in our process, where carboxylic acid modified MWCNTs were dispersed in acetic anhydride via ultrasonication prior to being charged to a prepolymerization reactor together with monomers and catalysts for esterification reaction at 130 degrees C-200 degrees C. The esterified mixture was then fed into a polycondensation reactor at 280 degrees C-320 degrees C. In this way, fully exfoliated MWCNTs were dispersed in the TLCP matrix at concentrations up to 0.3 wt%. Systematic studies show that well dispersed MWCNTs acted as "pseudo nucleation sites" for the nematic ordering in the adjacent TLCP melt. Thus the extrudates show a smaller core region and higher overall orientational order. Consequently, the addition of MWCNTs is not only effective in improving the mechanical stiffness but also toughness of the composites. For example, the 0.3 wt% TLCP/MWCNT composite shows a 62%, 135% and 145% increase in Young's modulus, tensile strength and toughness, respectively, in comparison with the pure TLCP. (C) 2012 Elsevier Ltd. All rights reserved.