Blood is composed of red blood cells, leukocytes and platelets, which are dispersed in plasma. These particulate components have different size and stiffness. In blood flow, red blood cells are found to flow in the center of vessels. On the other hand, leucocytes and platelets are observed in the vicinity of wall of vessels. This phenomenon is called ‘margination’. In this study, we simulate the behavior of binary droplet dispersion with size and surface tension differences under Couette flow, in order to investigate the mechanism of margination. We found that large floppy droplets migrate toward the center of channel, whereas small stiff droplets marginate near the wall of channel.