Recent development of organic semiconductor materials has led to the remarkable enhancement of mobility over 10 cm²/Vs in organic thin-film transistors (OTFTs). Despite these achievements, a thorough understanding of the correlation between molecular structure and charge transport properties is still not achieved. It is essential that we have a comprehensive picture of the structure-property relationship for these materials to further improve the mobility. However, this is a very challenging task since the resulting effects of molecule structure changes can be so manifold. In this work, we investigated the structure-property correlation by finely tuning the molecule structure of organic semiconductors and comparing the performance of TFT devices, aiming to deepen our knowledge of the structure-property relationship and to provide guidance for optimizing the synthesis of high-mobility organic semiconductors. Specifically, we have studied how the molecule length and molecule symmetry affect the charge transport properties of small-molecule semiconductors. We have also researched the effect of the donor unit length on the charge transport properties of donor-acceptor (D-A) copolymers. The work has at least clarified how these factors such as molecule length, molecule symmetry and the length of donor units influence charge transport in organic semiconductors. Our work thus is enlightening for people to understand the charge transport physics in OTFTs and it is also instructive for chemists to optimize the synthesis of high-mobility organic semiconductors.