In the past, many injection molding simulations based on the Hele-Shaw approximation to predict fiber orientation have been developed. Some researchers have also performed the flow/orientation coupling analysis by the Lipscomb constitutive equation of fiber suspension rheology, and demonstrated the close agreement between the coupled and decoupled solutions. At present, the 3D numerical techniques has been widely applied in engineering practices. The primary purpose of this paper is to understand the effect of the coupling and its importance especially in the 3D computation of the injection molded fiber composites.