A thermodynamic theory incorporating Flory-Huggins thermodynamics was developed to elucidate the observed rubber particle agglomeration phenomena in ABS molded under a severe condition. When the particle size of ABS is smaller than a thermodynamically stable domain size (D-s), rubber particle agglomeration can occur. Based on this criterion, rubber particle agglomeration can be explained semiquantitatively, especially for materials which do not have too insufficient graft level and nearly no compositional acrylonitrile mismatch. This finding suggests that the agglomeration results mainly from a driving force produced by thermodynamic incompatibility between components.