The wave-like flow mark phenomenon is one of the surface defects that can occur during the injection stage of the injection molding process. The generating mechanism of this phenomenon is not yet well understood. In this paper the flow mark generation mechanism is investigated by (i) directly observing the behavior of the polymer in the flow front and contact line areas under molding conditions when flow marks are generated, and (ii) studying the micro-geometry of the flow marks after molding. Specific attention was given to (i) the-point of origin of flow marks (relative to the moving contact line, as well as the history along the flow path), and (ii) the effect of thermal boundary conditions on the micro-geometry of flow marks. These findings and their significance in relation-to the flow mark generation mechanism are discussed. It is proposed that the flow marks are caused by nonuniform thermal contraction in the solidifying layer immediately behind the contact line.