Rotational diffusivity of rod-like macromolecules in concentrated solutions is severely affected by the entanglement of those molecules. The kinetic theory to describe this effect is recaptured to include the side wall effect of the test rod argument of Doi, which fumed out to be very important in case of strong extensional flow. The initial relaxation time τ₁ of the Kerr effect depends on the concentration ρ and the molecular weight M such that τ₁ ∝ ρ Ms instead of ρ²M⁷ given by Doi. which is good for the relaxation time for the final stage.

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