Strain-induced hardening behavior and flow birefringence were studied on copolymers of ethylene-tetracyclododecene (E-TD) and ethylene-norbornene (E-NB) of various compositions subjected to uniaxial elongation with constant Hencky strain rate between 0.01 and 1.0 s(-1) in the temperature range from +40 to +60 degreesC above their respective glass transition T-g. When compared at the corresponding temperature of, e.g. T-g + 50 degreesC, the E-TD copolymer exhibited stronger tendency of strain-induced hardening than the E-NB copolymer of the corresponding comonomer content. The stress optical rule (SOR) was obeyed for the E-NB copolymer in the whole range of its composition, while for the E-TD copolymer, the SOR was valid only for that of low TD content (< 30 mol%), but invalid for that of high TD ( greater than or equal to 30 mol%) content. The results suggest that the increase in the population of bulkier TD comonomer may be a molecular origin of the strong strain-induced hardening as well as a reason for the invalidity of SOR for the E-TD copolymer of high TD content. For the two copolymers that satisfy SOR, the stress optical coefficient C decreases exponentially with the comonomer content, and the tendency is stronger for E-TD than for E-NB, reflecting the comonomer size. The stress optical coefficient C vs. composition plots of the E-NB (block type) and E-TD copolymers are extrapolated at zero E limit to 5.5 X 10(-10) and 7.5 X 10(-12) Pa-1, respectively, which are expected to be the C for the NB and TD homopolymers if such polymers do exist.