In the kneading of glass-fiber-reinforced plastics by twin-screw extrusion, the use of a backward-mixing screw (BMS) element for melt mixing has been found to be effective in dispersing glass-fiber bundles. In this study, we use computational fluid dynamics (CFD) to investigate the mechanism for the effectiveness of BMS for glass fiber dispersion. CFD of BMS melt mixing revealed that there is high uniformity of transport in the direction of extrusion and efficient transportation occurs from low-stress to high-stress regions. These findings demonstrate that BMS melt mixing is highly effective at imparting stress to the overall resin passing through. In addition, there is a correlation between the incidence of nondispersion of glass-fiber bundles measured experimentally and the stress history minimum value. On the basis of the above factors, we propose a method for predicting the operating conditions in which the nondispersion of glass-fiber bundles is controlled. The operating conditions for controlling glass-fiber nondispersion can be determined for various different mixing elements and the possible production rate can be predicted. Predictions for the operating conditions were applied to BMS and a forward kneading disk element (FKD). The effectiveness of BMS for controlling glass fiber nondispersion is characterized for a broad range of operating conditions. (C) 2013 Society of Plastics Engineers