Macromolecular degradation of starch by heating and shear forces was investigated using a newly developed shear cell. With this equipment, waxy corn starch was subjected to a variety of heat and shear treatments in order to find the key parameter determining the degree of macromolecular degradation. A model based on the maximal shear stress during the treatment gave an improved prediction compared to existing models in literature based on specific mechanical energy input (SME) or shear stress multiplied by time (τ·t). It was concluded that molecular weight reduction of starch at the temperatures investigated (85–110 °C) is a time-independent process, during which the starch molecules are broken down virtually instantaneously by high shear force within time scales investigated. Consequences for design of shear based processes (especially extrusion) are shortly discussed.