In this study, the effect of injection molding conditions on the mechanical properties of polypropylene/liquid crystalline copolyester in situ composites was investigated. Molding conditions such as fill time, mold temperature, and mold thickness were varied to determine the most significant variables that influence mechanical properties. The mechanical property variations were correlated to the shear or extensional stress, which deforms a dispersed phase, and to the Graetz number, which indicates the influence of heat transfer. It was found that higher flexural moduli were obtained in thinner molds and that the maximum mechanical properties for a given mold thickness were obtained near a Graetz number of ten, This indicated that heat transfer was a controlling factor in the formation of in situ composites.