Eco-friendly and low-cost cellulose nanofiber paper (nanopaper) is a promising candidate as a novel substrate for flexible electron device applications. Here, a thin transparent nanopaper-based high-mobility organic thin-film transistor (OTFT) array is demonstrated for the first time. Nanopaper made from only native wood cellulose nanofibers has excellent thermal stability (>180 degrees C) and chemical durability, and a low coefficient of thermal expansion (CTE: 5-10 ppm K-1). These features make it possible to build an OTFT array on nanopaper using a similar process to that for an array on conventional glass. A short-channel bottom-contact OTFT is successfully fabricated on the nanopaper by a lithographic and solution-based process. Owing to the smoothness of the cast-coated nanopaper surface, a solution processed organic semiconductor film on the nanopaper comprises large crystalline domains with a size of approximately 50-100 m, and the corresponding TFT exhibits a high hole mobility of up to 1 cm(2)V(-1) s(-1) and a small hysteresis of below 0.1 V under ambient conditions. The nanopaper-based OTFT also had excellent flexibility and can be formed into an arbitrary shape. These combined technologies of low-cost and eco-friendly paper substrates and solution-based organic TFTs are promising for use in future flexible electronics application such as flexible displays and sensors.