In Injection molding of short-fiber-reinforced plastics, the flow during a mold filling process plays an important role in forming a flow-induced fiber orientation field. However, the flow field is also affected by the orientation of fibers. Dinh and Armstrong's rheological equation of state including an additional stress due to the existence of fibers was incorporated into the coupled analysis of mold filling flow and fiber orientation. The mold filling simulation was performed by solving the new pressure equation including in-plane velocity gradient effect and energy equation via a finite element/finite difference method as well as evolution equations for the second-order orientation tensor via the fourth-order Runge-Kutta method. The in-plane velocity gradient effect on the flow field was examined at the diverging flow around the point gate and the converging flow through the line gate. In the case of the diverging flow, the new terms due to the in-plane velocity gradient tends to increase the flow rate for a given pressure gradient. In other words, for a given flow rate, a smaller pressure gradient is required when the new terms are included than when they are not taken account. In the case of the converging flow, the opposite results were observed. It was found that the in-plane velocity gradient effect increases with the fiber volume fraction and aspect ratio.