WAXD，FTIR，TGA have been used to investigate the effect of heat treatment on the structural properties of chemically prepared polythiophene (PT)．The thermal stability of the high-molecular weight fractions of PT (PT2) are much greater than the low molecular weight fractions of PT (PT1) by thermogravimetric analysis．Heat treatment of PT|at 380℃，for 30 min (under N2) lend to significant incgeases in crystallinity and coberence length indicative of chain growth and extension．Heat treatment of PT1 is carried out only with low-temperature．The crystallinity is improved somewhat．Low-crystallinity and long chain facilitate the increased concentration doped and con-ductivity．Crystal structure of PT belongs tO monoclinic unit cell and individual chain stru-cture for polythiophene in which the thiophene units alternate on both sides results in a stra-ight-chian conformation．Fourier Transform Infrared (FTIR) spectra of unannealed and an-nealed PT

powder with AsF5 dopant at varying doping levels have been investigated．