The breakup of a radially thinning liquid sheet is described, attributing directly to the transition from laminar to turbulent flow. A radial liquid film flow is generated by a water issue from a thin cylindrical gap, which is constructed by the end of a circular pipe and the flat surface of a circular plate, to suppress the turbulence of the flow to the utmost. The liquid film flows radially outward on the plate, detaching from the peripheral edge of the plate, and then the liquid flows outward as a sheet. The sudden laminar-turbulent transitions of both the liquid film flow on the disk and the liquid sheet are presented, due to the velocity distributions inside them. When the turbulence induced by the transition of the liquid sheet increases, it causes to the breakup of the sheet. The breakup process through. perforation, ligament formation and drop formation due to the transition is also presented.