The organic semiconductor 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C8‐BTBT) is deposited on a single crystal (0001) Al2O3 (C‐sapphire) by a vacuum thermal evaporation, and effects of the layer thickness and preparation temperature on structural, morphological, optical, and electrical characteristics are investigated with X‐ray diffraction, atomic force microscopy observation, optical absorption measurement, and resistivity measurement with and without light irradiation. The C8‐BTBT layers possess the (001) out‐of‐plane orientation irrespective of the layer thickness and preparation temperature. The C8‐BTBT grains are growing up in direction parallel to the substrate surface keeping almost constant height, and the continuous layer is formed by the coalescence of the C8‐BTBT grains. The grain size of the continuous C8‐BTBT layer increases with raise in preparation temperature. The optical band gap energy could be estimated to be 3.32–3.35 eV regardless of the layer thickness and preparation temperature. The electrical resistivity decreases from 2.1 × 106 to 1.2 × 102 Ω cm with increase in the preparation temperature due to the increase in the grain size, and the light irradiation induce the drastical decrease to 42–28 Ω cm.