The goal of this article is to study the effect of atomizer exit area ratio on atomizer performance. The experiments are performed on the round liquid jet breakup of seven coaxial air-blast atomizers with water-air systems. The breakup morphology of liquid jet is observed first. The membrane-type breakup can be divided into two subregimes called bag-type breakup and membrane-fiber breakup, and a correlation of characteristic length on bag-type breakup regime is obtained. Then, we analyze the influence of atomizer exit area ratio on the breakup morphology of water-air jets. To obtain reasonable atomization morphology criterions, the atomizer exit area ratio is used to modify the Weber number and momentum flux ratio per unit volume. This method is found to be able to explain different experimental results in the literature, which is also close to the results of round liquid jet in cross air flow and secondary atomization. (c) 2014 American Institute of Chemical Engineers AIChE J, 60: 2335-2345, 2014