2D mica crystals (with thickness < 100 nm) obtained by mechanical exfoliation are incorporated for the first time into the design of organic thin-film field-effect transistor arrays and organic single crystal transistors as a gate insulator. The size of mica crystals can be up to the dimensions of an A4 piece of paper. All devices so fabricated exhibit high mobility and low operating voltage, indicating the high quality of the crystals and the great potential of mica crystals as a flexible, low-cost, transparent insulator for organic electronics.