It is important to investigate on the weld line of injection molded part because the weld line area could induce potential failure for structural application. In this paper, the weld line factor(W-L factor) was adopted to describe the strength reduction to the bulk strength due to the appearance of weld line. There were five engineering thermoplastics involved in this study, including neat PP, talc filled PP, TPO, ABS and PC/ABS. The experimental design was used to investigate on four main injection molding parameters (melt temperature, mold temperature, injection speed and packing pressure). Both the bulk and weld line samples were prepared at each process settings. The sample strength was obtained by the tensile tests under two levels of testing speeds(5mm/min and 200mm/min) and testing temperatures(room temperature and -30oC). The results showed that different materials had various W-L factors. For neat PP, the W-L factor was highest and independent to injection mold parameters. For other materials, highest W-L factor was obtained with the high melt temperature and high packing pressure. The mold temperature and injection speed had limited influence on W-L factor.