This paper presents a simple statistical model for calculations of the flow segment for polymer melts. The comparison of calculations with available data for five polymers demonstrates the model consistency. The model calculations show that the length of flow segment is generally more than the length of the Kuhn segment, and for high molecular weight polymers, the length of the flow segment is considerably less than the length of polymer chain between molecular entanglements. Our calculations also allowed for indirect evaluation of activation energies for monomer fluids, which are poorly determined from experimental data.