Organic field effect transistors consisting of Poly [N- 9′-heptadecanyl-2, 7-carbazole-alt-5, 5-(4′, 7′-di-2-thienyl-2′, 1′, 3′-benzothiadiazole] (PCDTBT) as active layer have been fabricated for detection of gases. The device exhibited highly selective response towards parts- per-million level of NO2 gas. For these devices response towards NO2 decreases with increasing gate bias due to the existing high density of free charges. A model has been developed to explain this variation of response with gate bias using transfer characteristics of the device (in air). This model enables to determine the density of holes released in PCDTBT layer on interaction with NO2 gas.