Block copolymer reduces particle size in immiscible polymer-polymer blends by suppressing droplet coalescence and by aiding droplet breakup through reduced interfacial tension. In this paper we separated coalescence from breakup and studied suppression of coalescence by block copolymers in a model blend composed of polystyrene (PS), high-density polyethylene (HDPE), and polystyrene-polyethylene (PS-PE) block copolymer. Coalescence was examined by monitoring particle size change vs shear strain while varying the shear rate, block copolymer concentration, molecular weight, and symmetry (coil size ratio of blocks). Even 0.5% PS-PE block copolymer significantly suppressed coalescence of HDPE droplets in a PS matrix. Assuming all of the PS-PE molecules were at the interface, the minimum concentration of block copolymers required to prevent coalescence under shear was found to be about 0.2 chain/nm2 at a coalescence shear rate of 0.1 s-1 for PS-PE with a molecular weight of 20-20 kg/mol. We found that this minimum concentration decreased with shear rate and with increasing molecular weight of PS-PE. We also found that, with the total molecular weight being the same, a PS-PE with a larger PS block suppressed HDPE particle coalescence more efficiently than one with a smaller block. These results indicate that steric repulsive interactions due to the presence of block copolymer at the interfaces are more important than those due to interfacial tension gradients.