In the cast film process a polymer melt is extruded through a slit die, stretched in air and cooled on a chill roll. During the path in air, while the melt cools, a reduction of both thickness and width takes place; obviously, thickness and width reductions are functions of draw ratio and stretching distance. Width distribution along the draw direction was measured on a iPP resin supplied by Montell as function of both flow rate and take up velocity. Final film width was found to decrease as take up velocity increase and, surprisingly, as extrusion flow rate increases. Thus draw ratio increase, attained by either lowering extrusion flow rate or by rising take up velocity, can lead to either enlargement or reduction of final film width. The process of stretching in air was modelled with coupled one-dimensional equations of continuity and motion based on work of Barq, Haudin, Agassant, and Bourgin (Int. Poly. Process. 9 (1994) 350) the crystallinity generation term, according to the Nakamura non-isothermal model, was included in the equation of energy lumped along the thickness direction. The polymer was considered as a viscous fluid (non-Newtonian), the apparent viscosity being function of temperature and strain rate. Furthermore, the effect of crystallinity on viscosity was somehow accounted for. The model equations were solved numerically. A modified expression of heat transfer coefficient with respect to the model of Barq et at. (1994) was applied leading to a better agreement between model predictions and data with reference to width distribution along the draw direction and final film thickness.