Extrusion conversion of waxy corn starch in a single-screw extruder was simulated by using a reaction engineering approach. A one-dimensional CSTR-in-series model system (a cascade of continuous stirred tank reactors) was employed. Fundamental relations, such as process kinetic equations for starch extrusion process, theological relations for starch dough, shear resistance properties of powdery starch, as well as other constitutive equations, were used in solving the system's mass and energy balance equations. Emphasis was made to model the conversion in the transition zone of the extruder where starch exists in a discrete powder form and its flow can not be described by theological equations of starch dough. Starch conversion induced by tribological shearing in this zone was found important in converting the powdery starch into a theological mass. Bulk density changes and powder-to-gel phase transition of starch under extrusion conditions were found critical in defining boundary conditions of functional zones in the extruder, which leads to applying appropriate equations for simulation. Profiles of temperature, pressure, shear stress, kinetic parameters, and the degree of starch conversion were calculated along the down channel direction of the extruder. The predicted degree of conversion were compared with the extrusion experimental results and reasonably accurate predictions were obtained. The potential application of this study was discussed.