In this article, the effects of nanofiber on the polymer chain orientation are studied using molecular dynamic techniques in the circular duct flow. The conformational tensor models, which have root in the molecular dynamic, applied to simulate the polymer chain orientation in suspension of nanofibers. The effects of nanofiber aspect ratio are also investigated on the polymer chain orientation. The results show that the polymer chains are less oriented along the flow direction for more nanofiber concentration and consequently, the extensibility of the polymer chains decrease. In fact, the nanofibers prevent to stretch of polymer chains along the flow direction. Moreover, the results indicate that as increasing the aspect ratio of nanofibers, the c(zz) components decrease but c(rr), c(theta theta), and c(zr) components increase. It reveals that the polymer chains are less oriented along the flow direction for higher aspect ratio of nanofibers, and consequently, the length of polymer chains is reduced. POLYM. ENG. SCI., 59:15-21, 2019. (c) 2018 Society of Plastics Engineers