Conjugated polymer nanowires with long-range order can significantly enhance charge carrier mobility. However, nanowires of P(NDI2OD-T2) with lengths up to the micron scale have not been reported yet due to fast backbone aggregation. Herein, we proposed to prepare P(NDI2OD-T2) nanowires through slow nucleation via side chain ordering first and then backbone planarization by enhancing the side chain interaction. For this purpose, two selective solvents were used, with bromonaphthalene (BrN) dissolving the P(NDI2OD-T2) backbone and toluene (Tol) dissolving the side chain, respectively. For BrN, the initial solution contained only P(NDI2OD-T2) unimer coils with both the backbone and side chain disordered. In the subsequent aging process, P(NDI2OD-T2) side chain ordering took place first then inducing backbone planarization. The resulting extended chains stacked slowly into nuclei upon the side chain interaction. During the final film-drying process, these nuclei grew into well-defined nanowires with lengths up to tens of micrometers and a width of 20 nm. Structural analysis revealed that the polymer chains aligned parallel to the long axis of the nanowire in an edge-on orientation. In contrast, in Tol, P(NDI2OD-T2) chains aggregated immediately into too many rod-like nuclei upon strong backbone interaction, which resulted in high-density small fibrils in the film. The results obtained herein reveal the subtle role of the backbone and side chain in nucleation and demonstrate that the nucleation pathway can be readily controlled using selective solvents, thereby manipulating the final film morphology of conjugated polymers.