Many workers in the past have found that poly(isobutylene) (PIB) exhibits viscoelastic properties distinct from other commonly known amorphous polymers. PIB has perhaps the most compact and symmetric monomer chemical structure among amorphous polymers, which leads to the expectation that it has a smaller capacity for intermolecular coupling and a lower degree of cooperativity of the local segmental motion. In the framework of the coupling model this expectation for PIB implies that it has a smaller coupling parameter which is sufficient to explain its various distinct viscoelastic properties as demonstrated in previous works. Up to now there is no direct determination of the coupling parameter for local segmental motion of PIB to confirm that it is indeed smaller compared with other amorphous polymers. Dielectric spectroscopy is inapplicable to PIB because the monomer has a negligible dipole moment. Mechanical spectroscopic measurements performed on PIB have not yielded unambiguous results because of the difficulty of isolating the contribution of the local segmental motion from the experimental data. In this work we have resorted to photon correlation spectroscopy and successfully obtained the correlation function of the local segmental motion of PIB which can be well fit by a Kohlrausch stretched exponential function. The stretch exponent, beta, has the value of 0.56. The coupling parameter, given by 1 - beta, has the value of 0.44 for PIB which is indeed significantly smaller compared with other amorphous polymers and consistent with the value we deduced indirectly from other experiments.