To determine the loop/bridge ratio of an ABA triblock copolymer, dynamic viscoelastic measurements were carried out using a polystyrene-block-polyisoprene-block-polystyrene triblock copolymer (SIS), a cyclic SI diblock copolymer (cyclic-SI), and a polystyrene-block-polyisoprene-blockpoly(2-vinylpyridine) triblock copolymer (SIP), which were carefully synthesized by the anionic polymerization technique. The three copolymers show similar microphase-separated structures having spherical domains of S and/or P. By blending cyclic-SI and SIS, samples having various loop/bridge conformation ratios were prepared, and the SIP sample was also used as the 100% bridged reference sample. From the dynamic elongational measurements for a series of blended samples and SIP, it was confirmed that the storage modulus (E) is the highest for the SIP, and it linearly decreases with increase of cyclic-SI content. From quantitative analysis of the results, we determined that the bridge fraction, phi(bridge), in SIS triblock copolymer is 0.93. Although this fraction is significantly higher than the theoretically predicted value and the reported value from dielectric relaxation measurements, the estimated bridge fraction in the present work includes the total contribution from bridged chains and looped chains with interlocked entanglements. Furthermore, it was confirmed that breaking stresses are proportional to the effective bridge fraction in the samples.