A polythiophene derivative with the simplest conjugated side chain, poly(3-hexy-l-enylthiophene) (P3HET), was synthesized by Stille self-coupling reaction. A comparative study of the newly synthesized polymer with poly(3-hexylthiophene) (P3HT), one of the most widely investigated optoelectronic materials, is presented. The effect of double bond (C=C) on the side chain toward thermal stability and optical and electronic properties was fully characterized by TGA, UV-vis absorption spectroscopy, photoluminescence spectroscopy, and cyclic voltammetry. The hole mobility of P3HET determined by the space-charge-limited current (SCLC) model is 6.7 x 10(-3) cm(2)/V s, which is comparable to P3HET with similar molecular weight and regularity and 1 order of magnitude higher than most conjugated-side-chain polythiophene derivatives. Polymer solar cells (PSCs) and field effect transistors (FETs) were fabricated respectively to exploit its potential applications in optoelectronic devices.