A theoretical and experimental investigation of relaxation dynamics is carried out for binary mixtures of entangled short and long chains of the same species. In disagreement with the previous theories of Doi et al. [Macromolecules 1987, 20, 19001 and Viovy et al. [Macromolecules 1992, 24, 35871, the new experimental data indicate that the terminal relaxation time associated with the long chains decreases systematically with the short chain length and with the weight fraction phi of the long chains for all the mixtures. These experimental observations have inspired the proposal of a new reptation theory to account for (1) the tube dilation (TD) due to the constraint release by short chains, (2) impedance of the long chain's curvilinear diffusion, and (3) enhanced contour length fluctuation, all resulting from the incorporation of the short chains of length N-S. The various increases of the long chain's relaxation rate with lowering phi for its mixtures with different short chain lengths are possible only if the long chain reptates in a dilated tube of a shorter contour length with a curvilinear diffusivity that depends nontrivially on p and N-S. The short chain's influence on the long chain's curvilinear diffusion, characterized by an impedance function that explicitly depends on N-S and phi, is demonstrated by comparison between the theory and experiment. Finally, the contour length fluctuation (CLF) is found to be enhanced in the binary mixtures and to produce additional concentration dependence for the terminal relaxation time; i.e., the CLF correction is larger at lower concentrations and for lower molecular weights of the long chains.