In this study, an effect of flow rate and melt temperature on film thickness in a film insert injection molding process is investigated for a precise manufacture of composite parts. Film insert injection molding is a considerably complex process where several phenomena occur inside a mold cavity and interact with each other. For example, remelting of the inserted film, asymmetric cooling, and residual stress development can cause low impact resistance and viscoelastic deformation along with severe warpage. While a change in film thickness is another important aspect to achieve a high-quality final product, there have not been many investigations. In this regard, the present study investigates flow rate and melt temperature as major parameters producing a change in the thickness of the inserted film. For the injection molding experiment, 50 μm thick polystyrene (PS) film is placed on the square cavity (30301.5 mm3) before the mold closing. Then, PS melt is injected into the cavities, forming the final PS/PS part. The flow rate and melt temperature are changed to investigate their effect on the film thickness in the final product. The film thickness for each product is measured along two flow paths – center and edge (12 mm from the center). Pressure and the bulk temperature inside the cavity are predicted by a numerical analysis. The film thickness is found to increase from the gate to the melt front along the center-path whereas only a slight change in the film thickness is observed along the edge-path. In both cases, the pressure inside the cavity decreases along the path, thereby showing a reduced compressive effect. However, the bulk temperature along the edge-path is found to be almost constant while the decreasing behavior of the bulk temperature is observed along the center-path. Therefore, the thickness of the inserted film can be considered to be more sensitive to bulk temperature than cavity pressure within the present conditions.