The effects of extrusion processing variables (such as melt tempeature, screw speed, and moisture content), Amylopectin/Amylose ratio as well as extent of starch conversion on the apparent viscosity of starch dough have been studied through the application of a capillary die and a slit die viscometers in conjunction with a single screw extruder, covering a shear rate range of at least two orders of magnitude. The results suggested that increasing melt temperature or moisture content reduced the viscosity. Both shear force and thermal energy contribute to starch conversion. Higher temperature and lower moisture content gave rise to a higher extent of starch conversion. 70% Amylose starch (Hylon 7) gave a higher overall viscosity and was gelatinized less than 98% Amylopectin starch (Amioca), which was attributed to the structural difference between these two corn starches. A simple linear modification of the temperature‐dependent constant and the moisture‐dependent constant in Harper’s viscosity model was shown to fit the data better. Expansion of this model including a power function dependence of viscosity on the extent of starch conversion further improved the prediction of viscosity, significantly in the case of high Amylose corn starch.