The weld or the weldline is formed, where the flow of molding compound confluents in a cavity. Adhesion at weld is imperfect. Strength of weld is low and is often lower than that of resin. However, the lowering mechanism of weld strength is not well understood yet. Few papers deal with fiber orientation and flow of molding compound in the vicinity of weld, therefore effective method for improvement has not been found. So, in order to clarify the cause of low weld strength, a weld was generated by placing an insert in a cavity and both weld strength and micro structure of weld were investigated. The main results obtained are as follows: Weld strength is very low and this came from the fiber orientation aligning along the weldline. The reacted resin with oxygen in air forms weld, which also lowers the weld strength. The weld strength is improved by preheating of BMC and by exhausting of oxygen in a cavity.