We have fabricated flexible field-effect transistors (FETs) using poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)], PCDTBT, as an active channel, poly(methyl methacrylate) (PMMA) as gate dielectric and biaxially oriented polyethyleneterephthalate (BOPET) as supporting substrate. The output and transfer characteristics of the devices were measured as a function of channel length. It has been observed that various OFET parameters viz. on–off ratio (~10⁵), mobility (μ ~ 10-4 cm² V^-1 s^-1), threshold voltage (V_{th ^~-}14 V), switch-on voltage (V_so ~-6 V), subthreshold slope (S ~ 7 V/decade) and trap density (N_it  10¹⁴ cm^-2 V^-1) are almost independent of the channel length, which suggested a very high uniformity of the PCDTBT active layer. These devices were highly stable under atmospheric conditions (temperature: 20–35^oC and relative humidity: 70–85%), as no change in mobility was observed on a continuous exposure for 70 days. The studies on the effect of strain on mobility revealed that devices are stable up to a compressive or tensile strain of 1.2%. These results indicate that PCDTBT is a very promising active layer for the air stable and flexible FETs.