Poly(trimethylene terephthalate) (PTT) and cationic dyeable poly(trimethylene terephthalate) (CD-PTT) were extruded (in the proportions of 80:20, 60:40, 40:60, and 20:80) from one melt twin-screw extruder to prepare four PTT/CD-PTT polyblended fibers. The molar ratio of 5-sodium sulfonate dimethyl isophthalate (5-SSDMI) for CD-PTT polymer was 2%. This study investigated the thermal and mechanical properties of PTT/CD-PTT polyblended fibers using gel permeation chromatograph, differential scanning calorimetry (DSC), thermogravimetric analysis, potentiometer, a rheometer, the density gradient, wide-angle Xray diffraction, extension stress-strain measurement, and scanning electron microscope (SEM). Melting behavior of PTT/CD-PTT polyblended polymers exhibited positive-deviation blends, and the blend of PTT/CD-PTT between 60:40 and 40:60 showed a maximum value of the melt viscosity. Experimental results from the DSC indicated that PTT and CD-PTT molecules formed a miscible system. The tenacity of PTT/CD-PTT polyblended fibers declined as the 5-SSDMI content increased. Crystallinities and densities of PTT/CD-PTT polyblended fibers were also found decrease as the 5-SSDMI content increased. The surface of PTT/CD-PTT polyblended fibers exhibited a uniform morphology, from the SEM observation. It indicated that PTT and CD-PTT polymers were identified to be a miscible system. Though there is a good interfacial adhesion between PTT and CD-PTT polymer, the mechanical properties of PTT/CD-PTT ployblended fibers were largely affected by the 5-SSDMI content.