In order to investigate the relationship between carrier mobility and crystal morphology of semiconductor thin films, the p-type conjugated organic small molecule 2,7 dioctyl [1] benzothieno [3,2-b] [1] benzothiophene (C8-BTBT) was adopted as the active layer of bottom-gate top-contact organic thin film transistor (OTFT), three kinds of processes, and dip-coating, ink-jet printing and vacuum evaporation, were used to prepare the C8-BTBT film. The growth behavior and crystallization of small organic molecules and their effects on the properties of OTFT devices varied with different processes. In addition, the effect of annealing treatment on crystallization of C8-BTBT was studied by XRD measurements. The results show that the films deposited by evaporation have higher crystallinity, high substrate coverage, and exhibit the Stranski-Krastanov (SK) growth mode, and the trap density of the corresponding device is the lowest, so that the mobility is as high as 5.44 cm²·V^-1·s^-1 and the on/off is more than 10⁶. In addition, annealing could severely destroy the crystallization of C8-BTBT. Therefore, controlling the growth behavior of the semiconductor layer, therefore improving the coverage and crystallinity of the semiconductor layer is an effective way to fabricate high performance conjugated small molecule OTFT devices.