Hexagonal micrometer-sized organic single crystals of a fused-ring thienoacene are grown controllably by physical vapor transport over a large area. A "two-dimensional organic-ribbon mask" technique is developed to fabricate organic field-effect transistors based on an individual microcrystal. The accomplished transistors exhibit mobility as high as 1.8 cm(2)V(-1)s(-1) with on/off ratios of typically > 10(7).