Poly(trimethylene terephthalate) (PTT) and acid-treated multiwalled carbon nanotubes (OMWCNTs) composites were prepared using melt processing, and effect of CNT loading on thermal and mechanical properties of PTT have been investigated. Addition of OMWCNTs showed no effect on thermal stability, crystallization, and melting temperature of PTT matrix. Wide angle X-ray diffraction also showed no change in the crystallite size of PTT on addition of OMWCNTs. The tensile testing results showed that tensile modulus and tensile strength of PTT increased significantly on addition of OMWCNTs. The reinforcing effect of OMWCNTs was further confirmed by dynamic mechanical analysis, which showed noticeable increase in storage modulus and shift in the glass transition temperature on addition of OMWCNTs. Scanning electron microscopy studies showed good interfacial interaction between OMWCNTs and PTT matrix.