The objective of this study was to realize flexible gas sensors using low cost solution processing such as drop casting. As active sensor material, a p-type organic semiconductor, αα-ωω-hexyl-distyrylbithiophene (DH-DS2T), was used. DH-DS2T based transistors exhibit high mobility together with a good air stability. As a chemical compound, DH- DS2T presents a good solubility in common organic solvent, which means thin films could also processed by solution fabrication. Sensor responses were studied by measuring the current through the semiconductor organic film as an ammonia gas concentration function (NH3: 25, 50, and 100 ppm). We demonstrate here that DH-DS2T has efficient sensor responses and leads to an efficient fully solution processed gas sensor on flexible substrate.