We investigated the characteristics of organic thin-film transistors based on alkylphenyl-substituted dinaphtho[2,1-b:2′,1′-f]thieno[3,2-b]thiophenes (21DNTT) called 4,4′-nP-21DNTT and 6,6′-nP-21DNTT. We found that fieldeffect characteristics were strongly influenced by the position of the alkyl substituent on the 21DNTT core. An optimal field-effect mobility of 0.46 cm 2 V −1 s −1 was obtained for the 6,6′-8P-21DNTT-based transistor.