Polymer chains in both dilute solutions and melts undergo cyclic rotation and retraction, which is known as tumbling, under steady shear flow. However, it is still not known how the individual molecules in melts rotate freely under the constraints caused by surrounding chains. In this work, a Brownian dynamics simulation is used to investigate the influences of the interchain interactions on the polymer chain motions in both dilute solutions and melts under steady shear flow. Compared with previous simulation studies, a greater number of similarities and differences between tumbling in dilute solutions and melts are addressed, and the results explicitly suggest the critical role of the entanglements in melts during shear flow. Three components of the gyration radius in different directions [flow direction (< R-gx(2)>), gradient direction (< R-gy(2)>), and vorticity direction (< R-gx(2)>)] are shown to exhibit different dependencies on the shear rate depending on whether dilute solutions or melts are being examined. However, the characteristic tumbling times tau(r) in both cases are proportional to (gamma) over dot(-2/3). The distributions P(T) of time T that the chains spend in each tumbling cycle show that both states exhibit an exponential decay of P(T/T-tau) in the high-T region. In the low-T region, P(T/tau(r)) in the melts with variable shear rates are coincident with each other, while P(T/T-tau) in dilute solutions show different shapes. With respect to the distributions of chain orientation, both cases show the same scaling relationships for shear rates and chain lengths. Based on these findings, main conclusions are as follows. The entanglements still restrict the evolutions of polymer chain configurations despite the number of entanglements decreasing with increased flow strength. The tumbling motion in melts can occur inside the tube, and the chain behaviors inside the confining tubes are rather similar to those in dilute solutions. (C) 2020 The Society of Rheology.