Packing structures in bulk single crystals and evaporated thin films and organic field-effect transistor (OFET) characteristics of two isomeric didecyl-dinaphtho[2,3-b:2',3'-f ]thieno[3,2-b]thiophenes (2,9- and 3,10-C10-DNTTs) were investigated. Although their packing structures in the bulk single crystals are different, the thin film structures are similar to each other, being consistent with the fact that both isomers afforded highperformance OFETs with similar maximum mobility of 8 cm2 V-1 s -1. On the other hand, device durability was influenced by the alkylation positions: excellent device characteristics were maintained for the 2,9-C10-DNTT-based devices, whereas the devices based on 3,10-C 10-DNTT showed negative Vth shift upon standing under ambient conditions. This characteristics change can be related to the morphological transformation in the 3,10-C10-DNTT thin film: the freshly deposited thin film consisting of the lamella packing motif was gradually contaminated with the bulk single crystal phase possessing characteristic π-stacking structure. From these results, it can be concluded that 2- and 9-potions are optimal sites for modification of DNTT core to develop new DNTT-based high performance and stable materials.