Aggregation‐structure formation of conjugated polymers is a fundamental problem in the field of organic electronics and remains poorly understood. Herein, the molar mass dependence of the aggregation structure of a high‐mobility conjugated copolymer (TDPP‐Se) comprising thiophene‐flanked diketopyrrolopyrrole and selenophene is thoroughly shown. Five batches of TDPP‐Se are prepared with the number‐average molecular weights (Mn) varied greatly from 21 to 135 kg mol−1. Small‐angle neutron scattering and transmission electron microscopy are combined to probe the solution structure of these polymers, consistently using a deuterated solvent. All the polymers adopt 1D rod‐like aggregation structures and the radius of the 1D rods is not sensitive to the Mn, while the length increases monotonically with Mn. By utilizing the ordered packing of the aggregated structure in solution, a highly aligned and ordered film is prepared and, thereafter, a reliable hole mobility of 13.8 cm2 V−1 s−1 is realized in organic thin‐film transistors with the moderate Mn batch via bar coating. The hole mobility is among the highest values reported for diketopyrrolopyrrole‐based polymers. This work paves the way to visualize the real aggregated structure of polymer semiconductors in solution and sheds light on the microstructure control of high‐performance electronic devices. The molar mass dependence on aggregation structure evolution and charge transport of TDPP‐Se is thoroughly examined by neutron scattering and complementary techniques. The rod‐like preaggregate with moderate length favors a highly aligned and ordered structure by bar coating. Hence, a reliable hole mobility up to ≈14 cm2 V−1 s−1 is recorded for thin‐film transistors using diketopyrrolopyrrole‐based polymers.