Disintegration mechanism of an annular liquid sheet with a core-gas flow has been studied experimentally. Air and water were used as the test fluids, respectively. Two nozzles with different annulus gaps were prepared to find out the effect of the liquid film thickness on the breakup mode qualitatively. The breakup modes were identified from the high-speed spark photographs. The backward illumination method was adopted for flow visualization. There exist at least three different modes of disintegration, namely, Rayleigh, bubble-breakup, and pure-pulsating modes depending on the air and water flow rates. Tentative maps representing those disintegration modes were provided with the liquid film Reynolds number and the aerodynamic Weber number taken as the dimensionless parameters.