Poly(thiophene) (PTh) nanoparticles with various sizes were directly prepared by Fe3+-catalyzed oxidative polymerization of thiophene with varying hydrogen peroxide (H2O2) concentrations. In the polymerization, the FeCl3/H2O2 (catalyst/oxidant) combination system was used as an initiator couple. Under the optimized reaction condition, percentage monomer conversions were > 90%. Willi increasing H2O2 molar concentration from 2.35 to 5.88 M, the average sizes of the PTh nanoparticles decreased from 51 to 12 nm, and their photoemission wavelengths shifted from red to blue color at the maximum excitation wavelength (lambda(UV)(max) = 400 nm). However, the molecular weights of all PTh nanoparticles were similar to 3500 g/mol. As a result, we could tune the emitting colors that resulted from variations of the effective conjugation chain length by manipulating the size of PTh nanoparticles.