Experiments were performed on the deformation and breakup of annular liquid sheet of nine coaxial twin-fluid air-blast atomizers with water-air systems by using a high speed camera. Due to the morphological difference, the annular sheet breakup could be classified into three regimes, which were bubble (shell) breakup, Christmas tree (cellular) breakup and fiber breakup. The roles of atomizer size and Rayleigh-Taylor instability in the annular sheet breakup were studied. The correlations on the instability wavelength and the size of cellular structure were deduced. The results showed that the dimensionless cellular size was proportional to We(-0.5), which were in good agreement with the experimental results. In order to have an overview of the different breakup mechanisms taking place over the wide range, we suggested categorizing these breakup regimes in a Weber number and dimensionless sheet thickness map. (C) 2015 Elsevier Ltd. All rights reserved.