Compared with typical organic thin film transistors (OTFTs), ultrathin film (≤ 10 nm) organic transistors offer an effective way to address some fundamental issues of OTFTs because of their intrinsic features.: i) The conductive channel in ultrathin film bottom-gate devices is close to the film surface which enables direct probing of charge transport in the conductive channel, ii) They possess unique advantages for construction of high-sensitive sensors, which contribute to multi-functional applications of OTFTs, and iii) Ultrathin films allow fabrication of highly transparent devices at low-cost.