The growth of highly conductive nanodomains in regioregular poly(3-hexylthiophene) (P3HT) thin films was visualized with a resolution of 20 nm by conductive atomic force microscopy (C-AFM). The current distribution images by C-AFM indicated that P3HT nanocrystallites with sizes smaller than 20 nm were dispersed in a matrix of disordered phase in the film spin-cast from chloroform; with thermal annealing, highly conductive spots appeared in the image. In contrast, the film spin-cast from o-dichlorobenzene showed a large number of conductive domains with dimensions from 20 to 200 nm. The conductive domains occupied 52% of the entire surface, and the current carried through the domains accounted for 66% of the total current, demonstrating that these conductive networks are essential for obtaining a sufficient macroscopic conductivity. The increase in macroscopic conductivity of the P3HT films was well explained from the viewpoint of nanoscale charge transport through the development of the conductive nanodomains. (C) 2013 Elsevier Ltd. All rights reserved.