Recently, a new type of internal-mixing twin-fluid atomizer has been used for advanced low-grade oil-fired boilers provided by the Mitsubishi Heavy Industries group. The present research focuses on the spray characteristics of two atomizer types, liquid straight flow (LSF) and liquid cross flow (LCF), with different liquid-gas mixing methods. In the LSF type atomizer, the liquid is fed straight along the atomizer center line and intersects with the gas fed from the surroundings, whereas the LCF type feeds the gas in the center and the liquid from the surroundings. The dynamic two-phase flow inside the atomizer affects the liquid breakup, spray formation processes, and spray fluctuations. First, two-phase Volume of Fluid simulations are applied to elucidate the liquid behaviors in the primary mixing zone inside the internal mixing chamber and at the exit port. At the exit port, the LCF type produces a thin liquid film around the inner wall with a nearly symmetric velocity profile, whereas the LSF type produces a thicker liquid film with a less symmetric velocity profile. Second, the differences in the spray characteristics are experimentally investigated. The LCF type produces a more uniform and thinner liquid film, resulting in finer droplets compared to the LSF type. Finally, experiments are performed to observe the rapid fluctuations in sprays immediately downstream of the exit ports using a high-speed video camera. The prominent frequencies of the fluctuations with the order of a few kilohertz are observed in the experiments, which agree with the simulation results.