The use of micrometer and nanometer-sized organic single crystals to fabricate devices can retain all the advantage of single crystal, avoid the difficulties of growing large crystals, and provide a way to characterize organic semiconductors more efficiently. Moreover, the effective use of such "small" crystals will be beneficial to nanoelectronics. Here we review the recent progress of organic single-crystalline transistors based on micro-/nanometer-sized structures, namely fabrication methods and related technical issues, device properties, and current challenges.