We characterized wall slip of tridisperse linear 1,4-polybutadiene on a silicon wafer in a parallel plate shear cell and tracer particle velocimetry. Tridisperse mixtures of fixed weight-average molecular weight M-w and varying number-average molecular weight M-n were prepared from nearly monodisperse polybutadienes. Their steady state slip behavior was examined at shear rates over the range of similar to 0.115 s(1). The results show that the slip behavior in the transition regime depends on M-n at constant M-w. This study also revealed that weakly entangled and unentangled chains in the mixtures influence wall slip differently: mixtures containing moderate amounts of weakly entangled chains exhibited enhanced slip while those containing unentangled chains did not. We explain this observation using the tube theory through the slip disentanglement mechanism proposed by Brochard and de Gennes in 1992 and conclude that the slip behavior is changed because of the force balance between the mobile and adsorbed chains and the reduced entanglement density and coil stiffness related to the incorporation of short chains of different lengths.