Simultaneous measurements of transient tensile stress sigma((epsilon) over dot(0);t) and birefringence Deltan((epsilon) over dot(0);t) were conducted under uniaxial elongation with constant Hencky strain rate (epsilon) over dot(0) on new olefin-copolymer of ethylene (E) and tetracyclododecene (TD) of three different TD content from 20 to 41 mol%. For the low TD content copolymer, the extentional viscosity eta (E)((epsilon) over dot(0);t)(= sigma((epsilon) over dot(0); t)/(epsilon) over dot(0)) showed only moderate increase with elongation time t or Hencky strain epsilon(= (epsilon) over dot(0)t) and the stress optical coefficient C((epsilon) over dot(0);t)(= Deltan((epsilon) over dot(0);t)/sigma((epsilon) over dot(0);t)) was nearly constant for the whole range of sigma examined. On the other hand, for other samples of high TD content (greater than or equal to 30%), we observed strong uprising of eta (E) Often referred to as strain-induced hardening, and rapid decrease in C with increasing sigma. As the copolymer chains were stretched out from the entangled state, the larger amount of the bulky TD groups in the latter may suppress the how-induced conformational change to take place and suppress the disentangling of the chains, leading to the increase in sigma and decreasing of C. (C) 2001 Elsevier Science Ltd. All rights reserved.