Two-dimensional solid-state C-13 magic angle turning (MAT) NMR spectroscopy, for measuring and separating the line shapes of chemical shift anisotropy (CSA) in respective C-13 species, has been applied to the study of dynamics in amorphous poly(2-hydroxypropyl ether of bisphenol A) (PHR). From the temperature dependence of the CSA line shapes, the flip motion of the phenylene ring is quantitatively analyzed; the phenylene ring undergoes a pi-flip motion with standard deviations, pi, of 25degrees and 30-35degrees at 27 and 90 degreesC, respectively. Accompanying the flip motion, the main-chain fluctuational motion is revealed from the temperature dependence of the CSA line shapes of the phenylene ring quaternary carbons. The cone angle of the main-chain fluctuation is 10-15degrees and 12-20degrees at 27 and 90 degreesC, respectively. On the basis of the main-chain motion, we propose a model to account for the coincidences of the frequencies and the activation energies of the phenylene ring, pi-flip motion and gamma-relaxation. In the model, the main-chain fluctuation is activated by the phenylene ring pi-flip motion in the neighboring chains, which would be the origin of the mechanical gamma-relaxation.