The mechanism of relaxation for an entangled linear chain of polystyrene is analyzed from the standpoint of the tube theory. Dynamic shear measurements were performed to obtain primary viscoelastic constants, $n_0$ and $Je^0$, in binary blends. Zeroshear viscosity $n_0$ and steady-state compliance, $Je^0$ were obtained by dynamic experiment. $n_0$ scaled with average molecular weight as $n_0$ $Mw^{3.4}$, and $Je^0$ increased strongly with polydispersity. And also, the dynamic experiments allow us to define a local tube renewal time and a terminal relaxation time for the relaxation of the longer chain in binary blends. The average relaxation time of nearly monodisperse fractions was proportional to the 3.4-power of molecular weight. In binary blends the terminal relaxation time of the shorter chain had the nearly constant value as its pure state. While, the longer chain experienced the so-called tube renewal by shorter chains constraining it. The time for the completion of local tube renewal was dependent on the weight fraction of the longer chains, being proportional to $w_2^2$. The terminal relaxation time for the longer chain was decreased as the $w_2$ decreased, which was caused by the tube expansion due to the local constraint release.
