Temperature dependence of solid-state C-13 nuclear magnetic resonance linewidths of six different amorphous polymers was examined around their glass-transition temperatures (T-g) under the conditions of magic-angle spinning and high-power proton dipolar decoupling (DD). The observed temperature dependence was explained by motional averaging over a dispersion of isotropic chemical shifts and an interference between local anisotropic motion and DD. On the assumption that the correlation time of the local anisotropic motion follows the Williams-Landel-Ferry equation, we found that the reference temperature approximately equals T-g+5 K. From this reference temperature, the fractional free volume of amorphous polymers at T-g is estimated to be 4.7%, which is much larger than that obtained from rheological experiments (2.5%). The difference is attributed to differences in the amplitude of motions detected by both measurements.