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Weldlines are fatal defects in many injection moldings of thermosetting resins and thermoplastics. Significant strength reduction by weldlines in thermoplastics is caused by poor adhesion, molecular orientation, and a V-notch effect. These factors have been little investigated in detail, in spite of being well known. In the present article, the V-notch effect on strength is discussed for two types of thermoplastics, polystyrene and polycarbonate. The depth of weldlines was obtained by milling on the weldline surface, and the fracture toughness was measured with a double edge notched specimen. Polystyrene, which was drastically weakened by weldlines, had relatively deeper V-notch regions and the fracture toughness was also reduced by weldlines. Although polycarbonate had the same fracture toughness as polystyrene, it had strong weldlines since the depth of weldlines was negligibly small.