In this study, we prepared poly(p-phenylene terephthalamide) (PPTA)/multiwalled carbon nanotube (MWCNT) composites by in-situ polymerization. The advantage of the in-situ polymerization method for the preparation of MWCNTs-incorporated composites is that the MWCNTs can be better dispersed in the polymeric matrices. Good interfacial adhesion between the MWCNTs and polymeric matrices was also achieved by functionalizing the MWCNTs. Inherent viscosity measurements and nuclear magnetic resonance (NMR) were used to characterize the structure of the PPTA/MWCNT composites. The morphology of the composites was also observed by scanning electron microscopy (SEM). The prepared PPTA/MWCNT composites exhibited improved electrical conductivity. The mechanically ground PPTA/MWCNT particles were investigated for use as an electrorheological (ER) material. The ER fluid was prepared from the PPTA/MWCNT particles with insulating silicone oil. The ER behaviour was confirmed using an optical microscope under an applied DC electric field of 1.7kV/mm.