Understanding the nucleation process is very important in growing polymer nanowires as it plays a decisive role in determining the crystal structure and size distribution. Polymer chain conformation determines whether the polymer chains could assemble to nuclei or not. Here, chain conformation and the nucleation process were controlled to grow 3,6-bis-(thiophen-2-yl)-N,N'-bis(2-decyl-1-tetradecyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole and thieno[3,2-b]thiophene (DT-PDBT-TT) nanowires. We changed the conformation of DT-PDBT-TT in solution and controlled the nucleation process by using a main solvent:cosolvent system. The main solvent was a low boiling point good solvent, and the cosolvent was consisted of two high boiling point solvents with different solubility. In fact, the chain conformation in the pristine solution was changed by choosing different main solvents (with H-bond, pi-pi or none interaction with the main chain) and temperatures. The absorption spectrum and TEM images showed that trichloro ethylene (TCE) was the best main solvent because it has H-bond interaction with the polymer main chain and DT-PDBT-TT conformation in it approaching unimer coil conformation, which is beneficial to grow nanowires. Mixed o-dichlorobenzene (ODCB) and anisole (AS) with different ratios were used to changing the solubility step by step to control nucleation process. Only when marginal cosolvent (ODCB:AS=1:1) was added, could it decrease the nuclei number and avoided the aggregates simultaneously. As the main solvent evaporated slowly, the nucleation and growth happened, leading to the nanowires formation. The resulting nanowires were about 63 nm in width and one to two microns in length. The width of the DT-PDBT-TT structures suggests that the polymer chains are oriented along the fibril axis. Our results indicated that there are two requirements for the nanowire formation, (1) the polymer chain conformation should approach unimer coil; (2) the nucleation density should be optimized, not too much and no aggregation happened. (C) 2018 Wiley Periodicals, Inc.