Organic semiconductor single crystal has excellent device performance due to its long-range ordered molecular arrangement, few defects and grain boundaries. It is considered as a candidate to realize the practicability of organic semiconductor devices. At present, researchers have developed a variety of methods that can be applied in the growth of organic single crystals. Micro-spacing sublimation is a method to prepare organic micro/nano single crystals by sublimating organic semiconductor molecules onto substrates under ambient conditions. However, when C8-BTBT was treated by this method, amorphous/polycrystalline islands instead of single crystals were obtained due to the low melting point of C8-BTBT. In this work, these amorphous/polycrystalline islands are successfully transformed into discrete single crystals by solvent vapor annealing. The morphology and structure of the obtained crystals were characterized by optical microscope, X-ray diffraction characterization and atomic force microscope. The results show that the prepared crystals have typical characteristics of single crystals.