We reported that PThTA nanoparticles were successfully prepared with Fe3þ-catalyzed oxidative polymerization and surfactant-free oxidative polymerization in an aqueous medium. A growth mechanism for the particle formation of the PThTA nanoparticles was suggested based on the initial PL spectroscopy and the number of particles in relation to the polymerization time. As the polymerization time increases, the emission color of the PThTA nanoparticles in the emulsion state changes from bluish purple to scarlet, with an estimated quantum yield of 3.22. Moreover, the maximum emission wavelength significantly shifted from 450 nm to 520 nm as the PThTA chain length, i.e. the molecular weight, increased as a function of monomer conversion. Our results provide new insights on surfactant-free oxidative polymerization and may serve as guidelines for the preparation of new conjugated polymer emulsion systems for potential optoelectronic devices. Future studies will be undertaken in new substituted polythiophene nanoparticles using surfactant free oxidative polymerization.