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Microcellular foams are foamed plastics characterized by a cell density typically greater than 10 9 cells/cm 3, with fully grown cells on the order of 10 m m, and a weight reduction ranging from 5 to 95%. An inert gas such as CO 2 or N 2 is usually used as the blowing agent. Compared to the unfoamed polymers, microcellular foams exhibit improved mechanical and physical properties, for instance, high strength-to-density, high toughness, high fatigue life, thermal properties, low dielectric constant, and reduced material weight and cost. In this paper, the influence of the saturation pressure and its drop rate on the microstructures of unfilled and glass filled ABS prepared in a batch process were investigated. Cell size distribution and cell density of foamed samples are also compared at various pressure drops and rates. The results show that although pressure drop has a significant effect on the nucleation at room temperature, its rate does not, however, affect the microstructure of the foamed samples, considerably.