The mechanism of drop formation from a thin hollow liquid (water) jet with a high-velocity core gas (air)flow was examined. Spray configuration was visualized and mean (cross-section-averaged) Sauter mean diameter (SMD), radial distribution of local SMD, and volume concentration of drops were measured for various liquid and gas injection velocities and annulus gap clearances. The atomization quality was improved with a higher flow, rate of atomizing gas; however, dependence of drop size on liquid flow rate turned out to be complicated. With an increase in liquid flow rate, the mean SMD first increases, then decreases, and then increases again, up to a maximum, followed by a decreasing range. Flow regimes (laminar or turbulent) and the momentum strengths of the liquid and the gas, and their combinations, were considered as the primary factors determining the overall atomization characteristics. The aerodynamic force was proven to be dominant in the first three ranges of liquid flow rate, while the hydrodynamic force predominates in the last range.