This paper deals with a fundamental study on the weld structure of thin-wall injection molded polypropylene plates. Effects of the mold temperature and plate thickness on the weld characteristics were investigated. In the experiment, several measurements were performed; distribution of the optical retardation, specific latent heat for fusion, V notch depth and width, and tensile strength of the specimen. The results indicated that the fountain flow of the melt occurred in the filling stage even with a plate thickness of 0.3 mm. It was also shown that molecular orientation in the weld region was relatively low. Thermal property measurement showed that crystallinity of the molded specimen was affected not only by the mold wall temperature but also by the plate thickness. Mechanical strength reduction by the existence of the weld was experimentally confirmed through the tensile test, and it was shown that the thin-wall polypropylene plate molded at high mold temperature broke from the weld region under small strain.