A facile and low-cost solution method is developed to fabricate large-area organic 2D single crystals on a millimeter scale. The 2D crystals are prepared on arbitrary non-crystalline substrates with thicknesses of a single molecular layer and perfect long-range crystalline order. High-performance OFETs are fabricated using the monolayer crystals as the semiconducting layer and the carrier mobility exhibits no thickness dependence, confirming the efficiency of the monolayer for charge transport.