It has been assumed that the block copolymers suppress droplet coalescence in immiscible polymer blends by preventing droplets from approaching each other. However, recent measurements of the minimum block copolymer coverage for suppressing coalescence did not support this mechanism. In this paper, we assume that the block copolymers suppress coalescence by preventing the rupture process of the matrix film between the droplets. By using stability analysis, the minimum block copolymer coverage was calculated. It has been shown that the minimum coverage is independent of the shear rate that is applied to the polymer blends but decreases with an increasing molecular weight. Such results qualitatively agree with the experimental observations.