The atomization mechanism of gas/liquid mixtures flowing through Y-jet atomizers has been studied by examining both the internal and external flow patterns. Air and water were used as the test fluids. Atomizers with transparent mixing ports were used to visualize the internal flow patterns, and the drop sizes were measured to examine the spraying performance. The liquid/gas momentum ratio was considered to be the appropriate parameter to describe both the internal flow and spray characteristics. A comprehensive physical model describing the atomization process is proposed; the process consists of ''direct collision,'' ''drop entrainment/deposition,'' and ''liquid film disintegration.'' The validity of the model is also checked with the drop size distribution data. It is confirmed that the trend of the circumferential film thickness variation within the mixing port coincides with variation of the spatial distribution of spray drops.