Weldlines occur at the interface of two adjacent flows of material behind an obstructive pin in a cavity in injection molding (meldline or hot weldline). Tensile strength of such "adjacent flow weldline" in injection molded polystyrene plates was evaluated by a mechanical step-by-step milling technique. The strength when the milling depth was 1/5 of the thickness from each surface was about the same and independent of the distance from the pin. In contrast, the strength without milling decreased once and then increased along the flow direction. This demonstrates that the strength of a weldline is predominantly dependent on the properties of the surface layer of the weldline. The depth of the surface layer was defined as the depth of the weld, D-w. D-w reduced monotonously along the flow direction and faded away with the V-notch, resulting in an increase of strength along the direction. On the other hand, it was considered that the farther from the pin, the flow-induced molecular orientation in the surface layer is greater. It caused a decrease of the strength along the flow direction. The sequence of decrease and increase in tensile strength of adjacent flow weldline is due to the complex effect of these two contradictory factors.