Simulation of the spray flow around a high-speed rotary bell-cup was carried out in order to determine the dominating factor in the decrease of transfer efficiency (TE) and to reveal the effect of shaping air and electric voltage on droplet behavior. Methods were proposed for quantitative evaluation of the factor decreasing TE and the trajectory angle, defined as the slope of the droplets trajectory to the x-axis. As a result, it was shown that the dominating factor in the decrease of TE is droplets that travel with the air flow around the target. Calculated results showed that the shaping air is effective to control small (<10 mu m) or large (>40 mu m) droplets, and the electric voltage is effective to control medium-sized droplets (10-40 mu m).