The tube model theory of Doi and Edwards based on de Gennes''reptation idea is extended to explain the viscoelastic properties of binary blonds in highly entangled stave composed of two linear monodisperse species with different molecular heights M₁ and M₂. A modifled tube model theory is proposed in order to incorporate the relaxation mechanism of constraint release by tube renewal in the polydisperse polymers. The relaxation of the high MW component chain by the concurrent reptation and constraint release is remodeled as the disengagement by pure reputation of an equivalent primitive chain. This equivalent primitive chain model enables us to obtain the longest relaxation times of blend components and to formulate a general stress equation. In addition to chose intermolecular interactions, the stress equation incorporates other modes of intermolecular chain dynamics such as chain length fluctuation and fast Rouse motions. Blending laws for viscoelastic properties can then be derived from the stress equation. The current theory is critically tested by comparing predicted values of the zero-shear viscosity and steady -state shear compliance with available experimental data on undiluted polybutadienes and polystyrenes. The comparison shows that the theory agrees well with the data over a wide range of blend composition and M₂/M₁ ratio.