The slip behavior of polydimethylsiloxane (PDMS) polymer melts flowing on weakly adsorbing surfaces made of short non-entangled PDMS chains densely end-grafted to silica has been characterized. For high enough shear rates, slip lengths proportional to the bulk fluid viscosity have been observed, in agreement with Navier's interfacial equation, and demonstrating that the interfacial Navier's friction coefficient is a local quantity, independent of the polymer molecular weight. Comparing the interfacial shear stresses deduced from these measured slip lengths to available friction stress measured for crosslinked PDMS elastomers, we further demonstrate the local character of the friction coefficient and compare its value to the monomer-monomer friction.Understanding the mechanical properties of polymer melts or polymer solutions usually needs a rheometer in which the friction on the plates is supposed to be known in order to decouple the interfacial properties and the bulk properties of the polymer. The simplest hypothesis is to assume the relative tangential fluid velocity to be zero. Historically, Navier introduced in a linear response approach a more general condition [1]: the shear stress at the solid-liquid interface should be proportional to the component of the fluid velocity tangent to the surface V :where k is an interfacial solid-liquid friction coefficient, assumed to be independent of the shear rate. This coefficient is usually converted into the so called Navier's slip length b = η/k. It represents the distance from the surface at which the velocity profile extrapolates to zero. The determination of slip lengths for simple fluids has been the subject of intensive experimental [2][3][4][5][6][7] and theoretical [8][9][10][11] researches over the last 20 years. The measured slip lengths lie between 0 to 50 nm and appear to be highly sensitive to tiny molecular details of the surfaces [3,5].Polymer melts can present huge slip lengths contrary to simple liquids. They thus are interesting candidates to quantitatively test Eq. 1 and possibly get rid of the molecular details of the surface. Indirect experimental evidences for a giant slip of polymers melts were provided by extrusion instabilities reported since the 40's [12], extensively studied by polymer rheologists [13]. In 1979, de Gennes proposed a simple explanation of such a huge slip, for polymer melts [14] flowing on ideal non-adsorbing surfaces. The physical idea is that k, which results from the local contact between monomers and the solid wall, should be independent of chain entanglements and chain length, while entanglements do control the polymer melt viscosity. The slip length should thus simply be proportional to the polymer viscosity: