This paper describes mechanisms of the breakup and the drop formation of a radially thinning liquid sheet, which is attributing directly to the transition from laminar to turbulent flow. A radial liquid film flow is generated by a water discharge from a thin cylindrical gap, which is constructed by the end of a circular pipe and the flat surface of a disk. The liquid film flows radially outward on the disk, detaching from the peripheral edge of the disk, and then the liquid flows outward as a sheet. Sudden laminar-turbulent transition occurs in the liquid sheet when the Reynolds number exceeds a critical value, resulting both in the perforation and the disintegration of the sheet under extremely high Reynolds number. The breakup process through perforation, ligament formation and drop formation is disclosed both by a zoom-in photography using a single pulse strobe and a steel camera and a high-speed photography using a multipulse laser and a drum streak camera. These photographs clearly show that droplets of a few tens micrometers are produced from the liquid sheet. Flow conditions for occurrence of atomization induced by the transition are also presented.