For injection molded long fiber composites, the Anisotropic Rotary Diffusion (ARD) model is significant in its ability to predict fiber orientation. The five-parameter ARD tensor was assumed to be a polynomial tensor-valued function depending on both the orientation tensor and the rate-of-deformation tensor. However, most of the fitted parameters are not convenient for wide usage. We, now, propose a new ARD tensor in which the probability ratio of the fibers lie along eigenvector coordinates obtained from the orientation tensor. This tensor is independent of the rate-of-deformation tensor and is indirectly to the orientation tensor. For validation, using the proposed ARD tensor to predict the shell-core structure for long fiber composites in injection molding is in good agreement with the related experimental data.