Suspension-polymerized polyvinyl chloride (PVC) is a powder consisting of particles 100 ∼ 150 μm in diameter, calledgrains. The grains have a unique hierarchical structure consisting of a skin and internal tight agglomerates of about 10 μm diameter. The agglomerate consists of primary particles of about 1 μm diameter, which in turn consist of domains of about 0.1 μm. The domain is composed of microdomains of about 10 nm. In this work, mechanical mixing of PVC with a miscible polymer, nitrile rubber containing 30% acrylonitrile (NBR-30), was conducted in an internal mixer and the significant events in the mixing period of 150 sec were examined with scanning and transmission electron microcopes. The process of mixing was very different from that of two amorphous polymers in that the skins of the PVC grains were first peeled off and then the agglomerates were broken down to the subsequent smaller particles, eventually becoming microdomains dispersed in the rubber. The NBR was found to be an effective medium for the breakdown of the particulate structure of PVC. The high storage modulus of NBR-30 and the strong adhesion due to the molecular mixing at the interface of the two polymers effectively transmitted the stress from the machine to the PVC particles. Also, the viscous heating resulting from the high loss modulus of NBR-30 brought about a temperature rise which weakened the interparticulate bonds and melted some microcrystallites of PVC, thereby facilitaing the disintegration of the PVC particles.