In the processing of micro cellular foamed polymer products, one of the most important factors is the dependence of rheology on the mixture ratio of the polymer and the gas used as the blowing agent. The formation of a cell by employing the thermodynamic instability of the gas in a one-phase condition is the principle behind microcellular foaming. The viscosity change of the polymer and gas mixture is important in designing dies and molds and affects the quality of the microcellular foaming product. In this experiment, the extruder was modified to mix polymer with gas under a one-phase melted condition. The mixed polymer with gas was run through various capillary die shapes to measure the change in polymer viscosity against the gas supply rate. This paper describes the effect of talc and the blowing agent on the viscosity in continuous microcellular processes, revealing major factors in determining the viscosity of a polymer and gas mixture.