A breakup model of a liquid jet issuing from a coaxial twin-fluid atomizer is proposed. The breakup of a liquid jet is modeled using the modified jet-embedding (JE) technique. An in tact core is separated into several control volumes and the conservation equations of mass and momentum of an individual element are solved. In the proposed model, the spray generated from the liquid jet surface has a distribution in which the droplet number decreases monotonically as the drop size increases. Along the liquid jet intact core, Sauter mean diameter (SMD) has a peak in the vicinity of the atomizer exit. The atomization rate decreases gradually along the center axis. The calculation tendencies of breakup length with air velocity are coincidental with those of the measurements, but the effects of air velocity on the breakup length in calculations are smaller than those of the measurements. As the air velocity increases, the total SMD decreases rapidly and the total atomization rate is almost constant. From comparisons of calculations to empirical equations, the calculations fall between two empirical equations, except at small air velocity. The airflow and the droplet behavior are calculated by a SIMPLER algorithm and PSI-cell model using a newly proposed breakup model. The radial distribution of the SMD has a peak which moves to the peripheral as it goes downstream. The droplet velocity in the center of the liquid jet is smaller than the air velocity in the vicinity of an atomizer exit. At a certain position, the relationship between the air and droplet velocities is reversed.