Measurements were taken of stress birefringence patterns in molten polymers flowing through geometrically complex channels. Six different flow channels were constructed for experiment, some representing the flow geometries of spinnerettes encountered in fiber spinning, and others representing mold cavities encountered in injection molding. All the flow channels had two glass windows, which permitted one to take photographs of the flow birefringence patterns of molten polymers with the aid of a polariscope. Quantitative information on the stress distributions in a flow channel was obtained, with the aid of the stress-optical laws, from the pictures taken of both isochromatic and isoclinic fringe patterns. The significance of flow birefringence measurement is discussed from the standpoint of die design for extrusion operation and mold design for injection molding operation.