P3HT and P3DDT were synthesized by the method of FeCl₃. Chemical structures were characterized by gel permeation chromatography(GPC) and 1H-NMR, thermal stability was studied by Tg. The absorption, fluorescence properties and the band gap of them in chloroform solutions and their spin-coating films were studied. The maximum Uv-vis wavelength of P3HT and P3DDT in chloroform solutions were 420 nm and 413 nm, respectively, and their corresponding emitted wavelengths are 602 nm and 573 nm.The maximum Uv-vis wavelength of their spin-coating films are 430 nm and 415 nm respectively, and their corresponding emitted wavelengths are 622 nm and 589 nm. Band gap data are 1.9 eV and 2.2 eV, respectively.